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Refractive index of optical material of powder is measured not as easily as a bulk material. Here, the prism coupling technique in combination with the immersion method is proposed to measure the refractive index of an optical material of powder. First, the powder material to be measured was dispersed in α-bromonaphthalene (C10H7Br) liquid to form a suspension mixture. The refractive index of the mixture, together with that of pure C10H7Br, was then measured at the wavelengths of 632.8, 1311, and 1553 nm using a commercial prism coupler. From the measured index values of pure C10H7Br and powder-dispersed mixture, the refractive index of the powder material was obtained on the basis of the Maxwell-Garnett model. Microcrystal powder from a LiNbO3 single-crystal, which has the known refractive index values, has been exemplified to demonstrate the method. The results show that the method is feasible with an accuracy of ±0.05. [ABSTRACT FROM AUTHOR]