A normal thiospinel CuIr 2 S 4 exhibits a temperature-induced metal-insulator (M-I) transition around 226 K with crystal structure transformation. In order to study gradual variations of the M-I transition by gentle chemical modification with increasing the number of 5d electron, we have prepared the polycrystalline samples of Cu(Ir 1 - x Pt x ) 2 S 4 and have investigated the electrical and magnetic properties of these samples. The features of the M-I transition in the electrical resistivity vary gradually with Pt concentration x. No structural transformation has been observed for x=0.25 and 0.30, where the abrupt jump disappears in the resistivity and the magnetic susceptibility around the transition temperature.