The molecular weight (MW) of chitosan added to the soil ecosystem has an apparent influence on the microbial biomass carbon and nitrogen (MBCN) from soil, soil respiration rate (SRR) and the mineral nitrogen e.g. NO₃-N and NH₄-N in soil, all of which increase with the amount of chitosan used. In the case of chitosan with lower MW, the MBCN, SRR and the NH₄-N increase rapidly first, and then gradually decrease. When chitosan with either medium or high MW was applied, no apparent change was observed on the MBCN and SRR at the initial period, starting from the 14th day they increase rapidly, followed by the decrease after 34 days. The changes of NO₃-N follow the same trend as that the MBCN and SRR exhibited. This is in contrast to the NH₄-N. The latter increases significantly at the beginning of the addition of chitosan with lower molecular weight to the soil, and then slows down. When chitoson with medium MW was applied, the changes of NH₄-N were directly associated with the concentration of chitoson. After chitoson with high MW was added to the soil, one cannot observe the considerable changes of NH₄-N until the 24th day.