The effects of solid particles and gas on cavitation inception caused by transient flow in an oil hydraulic pipeline have been studied. The existence of a stable gas pocket in the clearance space of particles in a cluster has been suggested as a new model of the cavitation nucleus. In experiments, the transient flow is generated by simultaneously releasing the pressure at both ends of a pressurized pipeline, and the pressure fluctuations are recorded. Teflon powder is used as solid particles, and N2 gas is used as a dissolved gas. The liquids used in experiments are turbine oil and distilled water. It has been definitely shown by experiments that Teflon powder in water plays an active role in forming the cavitation nucleus, but in oil it hardly does so. It has been concluded that the dissolved gas, or solid particles which are apt to be wet, have no considerable influence on cavitation inception in transient pipe flow.