The effects of the addition of silica nanoparticles (SNPs) on wettability of regioregular poly(3-hexylthiophene) (P3HT) organic semiconductor solutions on hydrophobic substrates and the carrier mobility in organic field-effect transistors (OFETs) made of these films are investigated. The dewetting of films made from P3HT solutions on hydrophobic substrates modified with octadecyltrichlorosilane (ODTS) is markedly suppressed after the addition of SNPs with phenyl surfactants. This enables us to fabricate continuous P3HT/SNPs films with high crystallinity by the conventional spin-coating technique, leading to higher mobility compared with P3HT FETs fabricated on non-modified substrates. Moreover, the addition of SNPs with larger diameters compensates for the degradation of mobility associated with the increase in the concentration of SNPs. Solution-processed P3HT/SNPs FETs on ODTS-modified substrates exhibit a field-effect mobility of 1.3 X 10(-2) cm(2) V-1 s(-1), which is almost comparable to that of P3HT FETs without SNPs (2.1 X 10(-2) cm(2) V-1 s(-1)). (c) 2015 Wiley Periodicals, Inc.