We report a newassay method on the measurement of the catalase activity, whose utilzation value is considered to be remarkable in the field of plant biochemistry. We named this method as a De-Coupling method. The essence of de-coupling method is the separation between the enzyme reaction and the indicator reaction. The optimum condition of the enzyme reaction was found to be following: on addition of 1 ml of substrate (H2O2: 20mM) to the fixture of the crude extract of enzyme (volume: 0.2 ml) and the ammonium phosphate buffer (volume: 1.8 ml; 0.93 M phosphate, 1.6M NHB, 2.5 M methanol, pH 7.0). After 30, 60 and 90 seconds of the enzyme reactions are proceeded, the reactions are terminated by 25% of tai-chloro-acetate (final concentration of 5%), respectively. The precipitated materials by tai-chloro-acetate was removed by the centrifugation (2000g, 10minutes). Formaldehyde produced in the enzymatic reaction was reacted with 2ml of acetylacetone (60mM). The indicator reaction -(HANTSCH REAKT10N)- in which lutidine is formed, was proceeded for 60 minutes at $25^{circ}C$.