PURPOSES: The performance of both string line and multi-sonic sensor systems were investigated with respect to achieving smoothness in a 5 cm-thick Stone Mastic Asphalt (SMA) wearing layer. METHODS: String line and multi-sonic sensor systems were applied in the leading and trailing lanes, respectively, with two-lane simultaneous paving. RESULTS: Two systems did not show any significant statistical difference in initial International Roughness Index(IRI). The multi-sonic sensor system produced smoothness similar to that by the string line system. CONCLUSIONS : The string line system was found to be very effective in eliminating roughness below a wavelength of about 2 m, confirming that a string line reference is the best system to obtain a smoother surface. A multi-sonic sensor system evidently demonstrated the capability of replicating a reference level and, partly showed a roughness averaging effect within the system length. It can further be concluded that the effect of smoothness of the underlying layer on the upper layer smoothness cannot be ignored.