The influence of the clearance gap between fin tips and the top surface of channel on laminar mixed convection heat transfer from longitudinal fins has been experimentally and numerically investigated. A conjugate analysis has been carried out in which the conservation of mass, momentum and energy equations for the fluid in the two fin enclosure are solved together with the heat conduction equation in the fin and the base plate. Average heat transfer coefficients were obtained for modified Grashof numbers 5 x 10(7) < Gr* <1 x 10(9), and dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.18 and clearance parameter was varied from c/H = 0.20 to c/H = 0.75, to determine the effect of the clearance gap. The velocity of fluid entering the channel was varied between 0.02 <= w(in) <= 0.23 m/s so that experiments were conducted for Reynolds numbers 250 < Re < 2300. The results obtained from this study show that the heat transfer coefficient increases with the decrease in the clearance parameter. As a result of experimental study, an empirical equation has been derived to correlate the average Nusselt number as a function of Reynolds number, modified Grashof number and fin spacing, fin height, tip clearance and fin length. This equation is also in agreement with the numerical results. (C) 2012 Elsevier Ltd. All rights reserved.