This paper investigated environmental effects of passive, air-breathing, and vapor-feeding direct methanol fuel cells (PAV-DMFCs). In these experiments, main experimental parameters are temperature ($30^{circ}C$, $40^{circ}C$) and relative humidity (25%, 75%). From these experimental results, the humidity plays a key role in terms of the water management at the cathode catalyst layer. During pure methanol feeding, the peak performance shows in relative humidity 25% and $40^{circ}C$. Under high humidity condition (relative humidity 75%), the water flooding of cathode blocks the oxygen reduction reaction at the cathode. The low humidity (relative humidity 25%) decreases the flooding of the cathode, in which the Warburg of relative humidity 25% decreases rather than that of relative humidity 75%. We concluded that rich water contents of the cathode slightly decrease activation overpotentials.