In this paper, basic theory on the measurement of refractive index n, absorption coefficient k and thickness d of absorbing thin films using a phase-modulated laser interferometer is presented. Relationships between n, k, d and the measured parameters P, Q, are introduced. The exprimental methods determining P, Q, and & as well as their self-correction are given. The evaluation of the phase difference is discussed in detail, also discussed is the experimental investigation of correctoin factor F.Three types of thin films (one weakly absorbing film and two absorbing films) are studied. The results are compared with those measured by an ellipsometer.