It is necessary to cool specimens during spraying in the case APS or HVOF, because process-induced heat rises the specimen temperature and leads to oxidation and spalling of coatings. A reasonable cooling just after spraying improves some properties such as microhardness, adhesion and cohesion of the coating/substrate system. In the modelling of specimen temperature and residual stress, it is necessary to know the flux distribution of the cooling jet like compressed air, CO2 liquid jet etc. Therefore, the evaluation of the flux becomes important. In order to measure and analyse the distribution of cooling flux imposed on the substrate, the theory of the inverse problem of heat conduction was applied and an experimental apparatus was designed to mesure the transient temperature. Because of its insensibility to the effect of measuring error, the conjugate gradient method, an effective method of inverse problems was chosen among several mathematical optimisation methods. The flux distributions of cooling jet can be estimated by using the measured data and a program written according to the conjugate gradient method.