How to measure the surface tension of surfactant using pendant drop. Measuring surface tension of cationic surface active agent is usually very difficult. Because, at first formating the drop, surface tension of cationic surface active agent is much higher than its true value. Because, at first formating the drop, surface tension of cationic surface active agent is much higher than its true value.And we cannot select which is most better value for measurement of surface tension of it. We work out a most resultful method by reducing the drop volume to change surface area, and we find out the surface tension will be changed synchronously. Result measured by this method is in accord with the result by using wilhemy method and spinning drop method.