A series of 10 binary blends of poly(9,9-dioetylfluorene) and regioregular poly(3-hexylthiophene) were investigated and found to be phase separated and to exhibit high field-effect mobility of holes. The mobility of holes determined from blend thin film transistors was in the range of 2.0 x 10(-4)- 1 x 10(-3) cm(2)/(V s) in blends of 10-80 wt % poly(9,9-dioctylfluorene). A concave-upward dependence of hole mobility on blend composition was found with a minimum in mobility around the symmetric blend composition (50 wt %). The field-effect mobility of holes in poly(9,9-dioctylfluorene) homopolymer could not be determined directly because of the barrier to charge injection from gold electrodes. However, from the composition-dependent field-effect mobility of holes in the series of binary blends, we estimated the hole mobility in poly(9,9-dioctylfluorene) homopolymer to be (6-10) x 10(-4) cm(2)/(V S), which is in good agreement with previous results obtained by the time-of-flight technique. These results demonstrate that relatively high field-effect mobility of charge carriers can be realized in blends of conjugated polymers of current interest for various plastic electronic applications.