Study of the equilivant lightness of chromatic object colors was reported by Ikeda et al. and Yujiri from the scotopic to the photopic illuminance range. In the photopic range, the results showed the dependence on the adapting illuminance to the Helmholtz- Kohlrausch effect âthe L/Y (lightness/illuminance) factor effectê. The model for predicting the perceived lightness of chromatic object colors proposed by the present authors has been modified to take into account the effect of the change of adapting illuminance. By use of the modified model by the present authors and the method for estimation of the equilivalent lightness in the mesopic range by Ikeda et al., the equilivalent lightness of object colors with five principal Munsell Hues and with various Munsell Chromas were estimated, for comparison with the results observed by Yujiri from 0.02 to 3000 lx.