A new analyzing method for determination of the thickness variation during crystal growth based on the interference intensity variation of the Michelson inference image has been developed. With this relative simple method, the crystal growth rate can be measured in a higher accuracy than the conventional analyzing method. This method can also be used to determine the growth rate on very flat surfaces and to investigate the growth rate fluctuation, which is related to the unstable crystal growth. An application example on studying the in situ time-dependent crystal growth rate of Ba(NO3)(2) from aqueous solution has been given.