A Instrument for measuring relatlve brightness of phosphor for fluorescent lamp is presented in this paner. lt uses the 0 / d illuminate condition.The defect of 0 / 45 or 45 / 0 illuminate condition is avoided. Effect of violet light in Hg-lamp and correction error of results are analyzed. The relation between correction coefficient and y color cooridinate of blue color phosphor is obtained. Measuring results of relative brightness of phosphor conforms to luminous flux of fluorescent lamp.