In the design of high speed machinery, designers must consider the problem of possible structural failure due to excessive dynamically varying stresses, which are induced by the varying external loads and internal inertia forces, in the links of the mechanism. A study of the dynamically induced stresses would indicate what values of the minimum permissible fatigue strength should be for safe mechanism operation. This paper investigates the nature of the stress fluctuation in high speed mechanism on the basis of the effects of both the loads and the friction. The latter is apt to be neglected in the usual analysis in spite of the fact that it is always generated in the operating machinery. The analysis is performed on the coupler of the slider-crank mechanism for illustrative purposes and the results are expressed in a non-dimensional form for design applications.