Abstract: The present paper deals with various identity conditions for motions of spatial mechanism, based on the use of conventional direction cosine matrices as analytical tools. The main advantage of applying identity conditions to the displacement analysis of spatial mechanism is that it enables us to avoid as much as possible the unwanted intermediate motion variables and thereby simplify the derivations of required displacement relationships. As an illustration of this approach, the displacement analysis of a seven-link 5R-2P spatial mechanism is briefly given.