Influence of microstructure changes caused by Equal-channel angular pressing (ECAP) process on corrosion behavior of pure magnesium in RPMI-1640 medium was investigated. The grain size of ECAPed samples (30µm) were greatly reduced compared with the grain size of the annealed sample (200µm). Then, the immersion test has been carried out for a certain period of time. It was revealed that mass loss of the ECAPed sample is larger than the as-received sample and the annealed sample. Thus, it could be considered that many crystal defects yielded by ECAP process reduced the corrosion resistance. However, the corrosion resistance has been improved to a certain extent according to reduction of crystal defects through the heat treatment at the recrystallization temperature or lower. In addition, the amount of gas generation of the ECAP sample after immersion test is larger compared with the as-received sample. Therefore, correlation between the amount of gas generated and the mass loss was confirmed. Based on qualitative identification of the elements by Energy Dispersive X-ray Spectrometry (EDS), the corrosion products of the sample surface after the immersion test has been estimated to be a kind of calcium phosphate. These above results have indicated the potential for fabrication of magnesium as bioabsorbable materials.