This paper presents a new method (the relative transmission fringe depth (RTFD) method) of determining the optical constants and the thickness of semitransparent thin films. It has the following distinctive features: high precision, simplicity, and a self-examining function for the measured results. In order to meet the needs of application and engineering design, the n-F diagram (F is a new parameter which expresses the relative transmission fringe depth) is drawn. From it, n, k and d of films can be determined conveniently and precisely. With this method, the constants and thicknesses of SnO₂:I heat mirror coatings, plastic films used in agriculture, and micasheets were determined. The results are in complete agreement with those obtained using a polarimetric meter. Also, the calculated T- lambda curves agree well with the T- lambda curves obtained using a spectrophotometer.