Abstract: With the increasing of labor cost and the upgrading of environmental protection requirements, it is urgent for product oil sales enterprises to adopt new technologies to realize automation of custody transfer metering in oil depots during oil product trade. In this paper, a kind of on-line densimeter which is used to measure automatically the oil density of oil unloading pipeline in the oil depot was developed based on the measurement principle of resonant cylinder type density measuring sensor. Then, the on-line densimeter was introduced in detail from the aspects of structural design, metering principle, installation mode, property test and etc. It is proved by the actual application that this new densimeter is up to the advanced international level, and it can measure precisely the density and temperature of the oil in the unloading pipeline, satisfying the custody transfer metering requirements of oil trade.