The plastic film-mulching cultivation has widely been used in China and is one of the important increasing grain production measures. To evaluate the effect of film-mulching on Soil Organic Carbon (SOC), soil Microbial Biomass Carbon (MBC) and Microbial Biomass Nitrogen (MBN) at different growing stages were determined. The objective of our research is to find the changes of soil organic carbon and the dynamics of soil MBC and MBN in brown earth and to evaluate the influence and mechanism of film-covering on soil organic carbon. The experimental site is located at the Long-term Soil Ecological Experimental Station (41°50'N; 123°34'E) of Shenyang Agriculatural University, liaoning, China. The soil is belonged to silty loam (Hapudic Luvisols). The fertilizing treatments used included: (1) control (CK), (2) nitrogen (N), applying 270 kg N ha-1; (3) manure (OM), applying farmyard manure 42 000 kg ha-1 (containing N 270 kg/ ha-1); (4)Manure+N and P (OM + NP),applying farmyard manure 42 000 kg ha-1 , 135 kg N ha-1 and P2O5 67.5 kg ha-1. Maize was continuously planted in the every plot (70m2/plot). One group of plots was mulched with plastic film after seeding every year (started at 1987), and the other was uncovered as normal cultivation. All treatments were 3 replicates. The samples from native vegetation were taken from the adjacent land with forest. All samples that collected between April and October of 2004 were immediately sieved to pass a 2-mm mesh and stored at 4°C for analysis. Microbial biomass C and N were determined by the fumigation-extraction method (Vance, et al., 1987). Organic C and N in the soil was determined by using Elementar Analyser (Elementar III,Germany). Soluble organic C was measured by a Total Organic Carbon Analyser (Elementar high TOC, Germany) as the difference between total C and inorganic C in the solution. SOC contents in tillage system were lower than that in native ecosystem. Compared with the normal cultivation, the SOC was increased by film-covering due to more residuals from higher biomass yield in these treatments, which suggests that we may reduce atmosphere CO2 concentration by the simple way. The amount of MBC in all film-covering treatments was higher than that in corresponding un-mulched treatments at all growing stages, suggesting that film-mulching may increase MBC. The amounts of MBC in all treatments peaked at the date of August 15 and then decreased due to the decrease of temperature in autumn. The clearly different trend in MBN between treatments with no OM fertilizer (CK, CK-C, N, N-C treatments) and treatments with OM fertilizer (OM, OM-C, OM + NP and OM + NP-C treatments) were found. MBN reached the highest value in CK-C on May, 28, in CK and N-C on June,24, respectively. It was suggesting that the earlier peak would occur in treatment with film mulching with no OM fertilizer application. MBN contents in the manure treatments were nearly two times higher than the corresponding treatments and all peaked at the date of August,15. These facts support the conclusion that application of organic manure could improve the levels of MBN (Wang, et al., 1996). MBN was considered as an important indicator of soil fertility (Boyle, et al., 1989). The fixation of fertilizer N by microbial biomass is affected by the applied fertilizer OM and N. it seems that it is necessary applying organic matter for sustaining soil fertility. Acknowledgements: This work has been supported by the Great Basic Development Programme of China (2005CB121108) and the National Natural Science Foundation (40571089)