Heat flux measurement in a wind tunnel is an indispensable requirement for the next generation re-entry vehicle design. Heat flux is usually calculated from a temporal change in surface temperature. Temperature-Sensitive Paint (TSP) is a global temperature measurement technique based on photochemical reaction. This technique has been already applied to a hypersonic shock tunnel, but poses some problems caused by TSP layer thickness. In this paper, the relationship between TSP layer thickness and measurement accuracy in heat flux was studied systematically. A test model is made of engineering ceramic, which coated with TSP composed of Ru(phen) ³⁺ as luminophore and polyacrylic acid as binder. The layer thickness was varied from 1 to 9 micro meters. Wind tunnel tests were conducted in the JAXA 0.44-m Hypersonic Shock Tunnel. The model was tested in Mach 10 hypersonic flow and heat flux caused by aerodynamic heating was measured. A comparison between TSP data and conventional thermocouple data showed that the relationship between the layer thickness and the temperature error is not a simple liner relationship predicted by the theory. The deviation from the theory is significant particularly for the cases with very thin coatings.