Entraining flow of high-speed unsteady fuel jet was successfully measured by means of particle image Velocimetry (PIV). Entrainment rate was estimated based on the integration of obtained velocity along the jet boundary, and the effect of fuel density (natural gas and hydrogen), injection pressure and ambient pressure was investigated. It was shown that large-scale vortex motion around jet tip makes a large contribution to the entrainment and the jet tip penetration increases with injection pressure. For hydrogen jet, the jet tip penetration and jet angle are larger than those of natural gas jet, however, the amount of air entrained air is approximately equal.