An experiment was carried out to measure the magnitude of the nonlinear coupling constant for a noncollinear surface‐wave interaction on Y‐cut LiNbO3. the resonant interaction was an upconversion interaction in which two surface acoustic waves with frequencies of 36 and 214 MHz interacted to produce a 250‐MHz wave. The 36‐MHz wave was launched with a phase velocity in theZdirection, and the 214‐MHz wave was launched with its phase velocity 10° off theZdirection. The magnitude of the coupling constant was found to be: (2.8±0.6)×109m/sec2W1/2.