CoSi is a semimetal and has a cubic structure of the B20-type, (space group:P2(1)3-T-4). We prepared CoSi polycrystalline films with about 130 nm in thickness using conventional rf magnetron sputtering method. Thermoelectric power shows a negative sign in the film specimens The maximum absolute value of thermoelectric power is about 45muV/K at 550K in the film specimen formed at sputtering powers of 45 and 180W for Co and Si targets, respectively. The temperature at which a maximum thermoelectric power is obtained is shifted from room temperature in bulk specimens to higher temperatures in the film specimens. When the sputtering power on the Si target increases, the resistivity of thin film increases in the temperature range studied. Resistivity of the film specimen formed at a sputtering power of 180W on the Si target shows negative and positive temperature dependence in the low and high temperature ranges, respectively. This transition temperature is consistent with that at which the maximum absolute value of thermoelectric power is observed.