The effect of rib orientation on flow and heat transfer in a four-pass square channel with skewed ribs in nonorthogonal-mode rotation was numerically studied by using omega-based Reynolds stress model (). Two cases are examined: in first case, the ribs are oriented with respect to the main flow direction at an angle of in the first and third passage and at an angle of in the second passage. The second case is identical to the first case with the ribs oriented at angle of in the three passages. The calculations are carried out for a Reynolds number of 25,000, a rotation number of 0.24, and a density ratio of 0.13. The results show that the secondary flows induced by ribs and by rotation combine partially destructively in the first and third passage of first case. In contrast, for second case, the secondary flows induced by ribs and by rotation combine constructively in the first passage, while the flow is dominated by the vortices induced by ribs in the third passage. In first case, a significant degradation of the heat transfer rate is observed on the coleading side of the first passage and on both cotrailing and coleading sides of the third as compared to second case. Consequently, the rib orientations at are preferred in the radial outward flowing passage with an acceptable pressure drop. The numerical results are in agreement with the available experimental data.