The measurements of density and velocity of ultrasonic waves at frequency of 2.5 MHz were made for coals of various rank (81.6-89.65 wt % of carbon). The ultrasonic suspension method was used for the ultrasonic wave velocity measurements. The simple empirical relation between ultrasonic velocity, density, and the mean atomic weight found by Pinnov for a wide range of crystalline materials was applied for coals. It was found that the value of log(density/velocity) can be interpreted as a measure of molecular weight of averaged structural unit. The effect of HCl-demineralization procedure on the size of averaged structural units was also discussed.