Abstract First P-wave arrival times recorded by the seismic network in the Ryukyu arc were analyzed in order to image the lateral variation in crustal thickness beneath the Ryukyu arc. The results indicate a low P n velocity (7.5 km/s) in the Ryukyu arc and a relatively high P n velocity (7.9 km/s) in the Okinawa Trough. The crustal thickness changes between the northern-and-central Ryukyu arc (range 23–27 km) and the southern Ryukyu arc (range 29–44 km). The crustal thickness of the former is consistent with that in the northern-and-central Okinawa Trough, suggesting a flat Moho stretching from the Okinawa Trough to the Ryukyu arc. This flat Moho extends uniformly over a wide area. In contrast, the crustal thickness in the southern Ryukyu arc is consistent with the characteristics of the crustal structure whereby the Moho becomes shallower in the vicinity of the axis of the Okinawa Trough and deepens remarkably with increasing distance from the axis in the southern Ryukyu arc. These differences would be caused by the difference in rifting style between the northern-and-central Okinawa Trough and the southern Okinawa Trough.