Presented herein are the results of the study that was conducted on the electrical characteristics of organic field-effect transistors based on poly(3-hexylthiophene), particularly the thickness and annealing temperature of their active layer is varied. The changes in field-effect mobility and current on/off ratio were explored. It was observed that both increasing annealing temperature from $60^{circ}C$ to $100^{circ}C$ and various concentrations influence the trade-off relations between the mobility and current on/off ratio. The surface morphology of the 2-${mu}m^2$ area with various thicknesses was scanned via atomic-forcemicroscopy(AFM) to verify the relationship between surface morphology, which is related to the thickness of the film, and device performance.