In this study, two sets of mobility conditions for a RSSR mechanism are developed to ensure the crank motion in the three dimensional four-bar mechanism. First set of the mobility conditions is the Grashof-type necessary conditions expressed only in terms of link lengths. The other set of the mobility conditions provides the necessary and sufficient conditions in analytic form accounting for the constraint of the deviation angle. For some special cases, the equations are simplified. The first set of equations may be useful in the initial design stage while the second set may be used for the final detail design. An example of mechanism is taken for the comparison of the analyses results.