Soil organic carbon (SOC) plays a key role in soil fertility and atmospheric CO2 concentration. Our previous work found that tillage induced changes in aggregate stability and pore structure were major factors contributing to the effect of tillage system on SOC. Effects of earthworms and residue on SOC vary among tillage practices, but we don't clearly know how earthworms, residue and tillage interact to affect SOC. A further study was carried out by adding C-13 labeled maize residue and earthworms to undisturbed soil mesocosms from conventional tillage (CT) and no tillage (NT) plots and then incubating for 180 days at 18 degrees C. SOC, C-13, and soil aggregate were measured prior to and after incubation and pore structure was measured after incubation. Newly added residue had the same decomposition rate in CT and NT. Earthworms reduced the SOC content in CT, and the newly added residue reduced the SOC in NT. The earthworms increased average pore diameter of > 500 mu m, and decreased the pore number of 100 - 500 mu m in NT. The newly added residue improved the SOC by increasing the MWD in CT. The earthworms could change the SOC by increasing the volume fraction in NT. This result is helpful in understanding how the earthworms and residue affect the SOC under different tillage practices and provides direction for developing strategies to increase soil fertility and mitigate emission of CO2 to the atmosphere.