Nerium oleander L. (Common oleander, Apocynaceae), is a widespread toxic plant of Mediterranean region and was reported that this plant have both herbicidal and insecticidal activities. Therefore, it's essential to monitor the effects of leaves on soil microbial activity before using it commercially in agricultural fields. In this study, oleander leaves were mixed with soils at doses of half (L0.5), same (L1) and double (L2) of soil organic carbon level and carbon mineralization (Cmin) was measured for 30 days at 28 degrees C under 80% and 60% of field capacity (FC) of control soils. Soil cellulase and xylanase activities with counts of aerobic bacteria and fungi were determined on 15th and 30th days of Cmin. Highest cumulative carbon mineralizations were also observed at L2 dose. Control soil was significantly different with L1 and L2 (P < 0.05) under 80% FC but there were no differences under 60% FC at the end of incubation period. Carbon mineralization was generally, significantly and positively correlated with microbial populations and enzyme activities under 80% FC (P < 0.05). Highest bacteria counts were obtained in L 1 on 15th day and in L0.5 on 30th day while fungi counts were highest in L1 on 15th day and in L2 on 30th days under both moistures. Soil enzyme activities generally were enhanced all treatments and highest at L2 dose. Our results showed that addition of oleander leaves to the soils under different field capacities generally stimulated both activities and populations of soil microorganisms.