A pot experiment was carried out at Agriculture Faculty, Benha University, Qalubia Governorate, Egypt, during April to August in 2014 to evaluate biodegradability of two types of biodegradable plastic films produced from polylactic acid(Bi-OPL^® and Ecovio^®) compared with nondegradable one produced from polyethylene that incubated in the soil for five months. As well as, the effects of degradation compounds on microbial activity were debated. The obtained data revealed increasing amount of evolved carbon dioxide, microbial biomass and dehydrogenase activity from the soil amended with degradable plastic by increasing incubation period. There are significantly increase in the previous parameters in the soil amended with degradable plastic compared with nondegradable one. Bi-OPL^® films that is produced from pure polylactic acid has higher weight loss and lower tensile strength compared with Ecovio^® which consists of 45% polylactic acid (PLA) and 55% Ecoflex. While, polyethylene films have slight decrease in weight during incubation periods. It worthy to reported that increasing weight loss values and decreasing tensile strength values of degradable plastic films were related to the higher values of CO₂ evolution, microbial biomass and dehydrogenase activity. Accordingly, it may be depended on the previous parameters to evaluate biodegradability of PLA.