The laminar burning velocity of pure and diluted high-temperature propane air mixtures is extracted from the planar flames stabilized in the preheated mesoscale diverging channel. The experiments were carried out for a range of equivalence ratios of 0.7 <= Phi <= 1.3 and mixture temperatures of 370-650 K. The effect of dilution using CO2 and N-2 gases (up to 40%) on C3H8-air burning velocity is also studied. Experiments complimented with computational studies of experimental conditions confirm that the stabilized flames were planar in both transverse and depth directions, and the burning velocity with heat flux in the present case is nearly equal to the adiabatic burning velocity. The detailed uncertainty analysis shows the accuracy of the present measurement within +/- 5%. Computational predictions of burning velocity and detailed flame structure were performed using PREMIX code. The present experiments are successfully validated against existing experimental and computational results. The peak burning velocity was observed for slightly rich mixtures even at higher mixture temperatures. The minimum value of the temperature exponent is observed for slightly rich mixtures. The burning velocity was observed to decrease with the dilution of inert gases. The addition of CO2 shows a pronounced decrease in the burning velocity, as compared to N-2.