To calculate the complex conductivity &sgr;=&sgr;1−j&sgr;2, and thus the surface resistanceRsand penetration depth &lgr;L, of highTcsuperconducting thin films from microwave power transmission measurements, new expressions are derived taking into account the film thickness. Numerical examples are given to show that, compared with the calculations which neglect the film thickness, corrections in &sgr;1, &sgr;2,Rs, and &lgr;Lare a few percent for film thickness ranging between 0.65 and 0.46 &lgr;Land that corrections in &sgr;1and &sgr;2are greater than those inRsand &lgr;L. Using the newly derived expressions, maximum errors inRsand &lgr;Lare estimated to be about 30% and 3%, respectively, if errors between −10% and +10% are tolerated in the measurements of the magnitude and phase shift of microwave transmission.