The optic（relative） density in coal is an important index to the physical properties of coal.The principle of determination method of the optic（relative）density in coal was expounded；the determination conditions were studied, the factors affecting the accuracy of the test results,such as wax temperature， wax effect and sodium dodecyl sulfate solution were discussed.The most suitable wax temperature was 70~75 ℃, concentration of sodium dodecyl sulfate solution was 0.05%.