Oxidative potential (OP) of PM_2.5 is an emerging health indicator representing its ability to induce oxidative stress and cause adverse health effects. We examined pollution levels, temporal variations, and key constituents of PM_2.5 OP by DTT assay in both indoor and outdoor environments in Nanjing, China, for over one year. Outdoor OP_M (mass-normalized OP characterizes toxicity) and OP_V (volume-based OP characterizes overall oxidative burden) in Nanjing were at a medium level compared to results reported for twenty-seven cities. Although PM_2.5 mass concentration consistently decreased during outdoor-to-indoor transport, OP_M varied by a factor of up to 2 in either direction, indicating a change of PM_2.5's ability to disrupt oxidative-reductive balance. Temporally, both outdoor and indoor OP_M exhibited a significant seasonality pattern (P < 0.01) as autumn > summer > spring > winter. Outdoor and indoor daytime-nighttime OP_V and OP_M are fluctuating within two-fold range. In addition, the change in water-soluble Fe had the highest correlation coefficient (P < 0.05) with ΔOP_M (ΔOP_M = OP_{M, in}-OP_{M, out}) among constituents measured here. Our results suggest that development of mitigation strategies take indoor PM_2.5's OP into account, instead of outdoors only, since they differ.