Three-dimensional numerical computation was performed for the developing region of flow and thermal fields in finned channels. The channel studied here has a heated region of finite length to which plate fin arrays are attached in order to improve its heat transfer performance. The performance was evaluated using Nu_L^^*, an apparent Nusselt number corresponding to the heat transfer rate per unit heated area. The effect of fin height, fin pitch, fin length and Reynolds number on Nusselt number was examined parametrically. As the fin height increases with the fin pitch kept constant, Nu_L^^* increases monotonically. The maximum value of Nu_L^^* was obtained when the top of the fin reached the top wall of the channel. When the fin pitch was changed with the fin height kept constant, the numerical results demonstrated two tendencies. In the case with clearance between the fin top and the wall, there existed the optimal fin pitch leading to maximum Nu_L^^*. In the case without clearance, Nu_L^^* increases with decrease of fin pitch. The optimal fin pitch gradually decreased with decrease of fin length and/or with increase of Reynolds number. The ratio of the j-factor to the f-factor was also obtained to evaluate the performance of the fins.