Carbon fiber composite stiffened panels have been widely used in modern aircraft. Ultrasonic phased array C-scan technology is one of the effective methods for this laminated structure in-service. In practice, results for the same damage of multi-crystal phased array C-scan may be inconsistent with different detecting directions. However, this phenomenon does not occur by the conventional single crystal C-scan testing. This paper analyses these inconsistencies from surface curvature and defect characteristics and presents recommendations to avoid the occurrence of these uncertainties.