The surface tension of solutions of synthesized ammonium caprylate and ammonium caprate was measured by the ring method. The changes with of surface tension of solution of ammonium caprylate showed evidently that the salt had the critical micelle concentration(c.m.c.) at $30^{circ}C$. The c.m.c. values of above two salts were determined from the change of surface tension of solution with concentration at various temperatures. The temperature dependence of the c.m.c. and the influence of addition of excess ammonia on the c.m.c. were also investigated.