Nowadays, layer based rapid prototyping (RP) technologies have been used not only to make product prototypes but also to manufacture small batch functional parts. For RP parts fabrication, build directions play an important role. There were many reports about optimal build directions for RP part fabrication, yet factors influencing build directions of mechanism joint have not been studied. This paper will look into the factors affecting build directions of mechanisms. A slider-crank mechanism is used as an example as it is the fundamental mechanism for many machine designs. Apart from factors affecting mechanism build directions, guidelines for mechanism joint design are also discussed.