Effects of specimen thickness and orientation on creep and stress-rupture properties of two advanced single crystal alloys were investigated. The alloys were CMSX-3, a derivative of Mar-M247, and a new single crystal alloy, designated as Alloy X, which is based on modification of IN-792 composition. Section thickness effects were ascertained utilizing four different specimen geometries, including 0.250 in. diameter bars and 0.020 in. thick specimens. The stress-rupture lives of both materials degraded with section thickness at stress levels less than 40,000 lb/in.2. The life degradation was approximately 3X at 20,000 lb/in.2. Investigations into the effects of orientation on stress-rupture properties indicated that for Alloy X the <110> oriented specimens suffered a loss of approximately 3X in stress-rupture lives compared to <001> oriented specimens. On the other hand, essentially no difference was observed in the stress-rupture lives of <001> and <110> oriented specimens of CMSX-3 alloy. Time for 1% creep was essentially unaffected by section thickness or orientation in both alloys.