This paper presents a dynamic analysis of a tension leg platform subjected to wave forces. The scope of this paper is twofold. One is to compare two methods for calculation of wave forces. The first method uses the modified Morison equation on all members, while the second method utilizes the diffraction theory on large diameter members and the modified Morison equation on small diameter members. The other is to examine the dynamic characteristics of a tension leg platform by parametric studies. Analysis have been carried out using a imaginary structure of a tension leg platform with a triangular shape.