A combined effect of microporous layers (MPLs) on direct methanol fuel cells (DMFCs) is investigated. From the distribution of the outstanding carbon loading combinations of the cathode MPL and anode MPL as well as the evolutions of polarization curves, a combined effect in which the contributions of the two MPLs interdepend is observed. A further discussion indicates that either MPL in DMFCs is of double roles: a side role of obstructing mass transfers and a main role of adjusting an interaction between the anode and cathode. It is inferred that it is the combination of the two roles that produces the combined effect.