This paper is concerned with the kinematic nonlinearity measure of parallel kinematic machine tool (PKM),which depends upon differential geometry curvature.The nonlinearity can be described by the curve of the solution locus and the equal interval input of joints mapping into inequable interval output of the end-effectors.Such curing and inequation can be measured by BW curvature.So the curvature can measure the nonlinearity of PKM indirectly.Then the distribution of BW curvature in the local area and the whole workspace are also discussed.An example of application to the interpolation accuracy analysis of PKM is given to illustrate the effectiveness of this approach.