The air entrainment process of a compressed natural gas transient fuel jet was investigated in a constant-volume chamber using Schlieren and particle image velocimetry (PIV) techniques. A new method of calculating air entrainment around a gaseous fuel jet is proposed using Schlieren and PIV imaging techniques. This method offers an alternative to calculation of an alternative to calculation of entrainment using LIF technique in gaseous fuel jets. Several Jet-ambient pressure ratios were tested. In each test, nitrogen was used to fill the chamber as an air surrogate before the jet of natural gas was injected. Schlieren high speed videography and PIV experiments were performed at the same conditions. Schlieren mask images were used to accurately identify the jet boundary which was then superimposed onto a PIV image. Vectors adjacent to the Schlieren mask in the PIV image were used to calculate the spatial distribution of the air entrainment at the jet boundary. The effects of ambient density and injection pressure on the air entrainment and contour shape at various parts of the jet are investigated. Results indicate that increase in injection pressure increases the entrainment around the jet. At the same time it was found that ambient density also has a similar relation on entrainment. The air entrainment distribution in gaseous fuel jets was found to be highly sensitive to the jet boundary definition, highlighting the importance of accurately defining the jet boundary.