To investigate the effects of riblets on the onset of Goertler vortex flow transition, an experiment was carried out on a ribletted concave surface of 2.0 m radius of curvature in a perspex curved rectangular duct connected to a low speed wind tunnel. Semi circular section riblets with height (h) of 1.0 mm and spacing (s) of 2.2 mm (that is, with h/s = 0.48) were used. The investigation was carried out by means of measurements using a single sensor hot wire anemometer. The boundary layer transitions were detected by turbulent intermittency measurements using the modified turbulent energy recognition algorithm (M-TERA) method. The experimental results obtained show that for free-stream velocities of 3m/s and 6m/s, boundary layer transition on the ribletted concave surface starts and ends slightly earlier than on the corresponding smooth concave surface. However, for free-stream velocities of 9m/s and 12m/s, the boundary layer transitions (both the starts and the ends) on the ribletted concave surface were delayed compared to the Goertler vortex flows on the corresponding smooth concave surface.