Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C ₆₁ -butyric acid methyl ester (PCBM) blends have been demonstrated to form highly efficient polymer photovoltaic devices. In this letter, the time-of-flight technique is used to investigate the influence of composition and heat treatment on charge transport properties of P3HT and PCBM blends. The transport of electrons and holes both display a transition from dispersive to nondispersive and return to dispersive again as the percentage of PCBM increases. A balanced mobility of both electron and hole is obtained at a composition of 1:1 weight ratio, and it is nearly independent of the electrical field in the range of our test. The increase in carrier mobility is attributed to the formation of a more-ordered structure in the blend. This structural ordering is further enhanced by slowly evaporating the solvent during film formation which results in additional increase in carrier mobility. However, no such effect is observed in thick films (∼200 nm ) , indicating the presence of such high-degree ordering due to heat treatment.