By impacting a concave dual-layer liquid(with the oil and the dyed water)surface in a test tube,a compound jet with a water core and an oil shell is generated.The velocities of the jet's head(Vj)and the oil-water interface(Vj')are experimentally determined against the initial thickness and the viscosity of the upper layer liquid.The results show that the oil-water interface can be roughly regarded as a pure material surface,whose velocity can be determined from the jet velocity and the initial location(or the film thickness).Furthermore,the effective density,viscosity and surface tension of the compound jet are used to evaluate the jetting velocity.The previous scaling relation for a single liquid jet can be used to give a good prediction with the effective quantities.The results of this paper can be used for the compound jet/droplet preparation.