A transient gas jet and its flame are the most fundamental phenomena of a transient spray and its flame breaking out in a CI engine and an SI engine with the direct injection system. In the case of CNG and LNG engines, the fuel itself is just gaseous state. The 2-LIF technique was applied to the transient gas jet to obtain the mixing process between the surroundings and it, and the simultaneous application of LII and LIS techniques were applied to the transient gas jet flame to obtain the soot formation process.The conclusions drawn from the experiments are: (1) The vortices structure of the jet has the close relation to the tendency of the relative fuel concentration. (2) At the upstream, the entrained surroundings are very small near the jet axis, however, they increase as the location goes to the jet periphery. (3) As the location goes farther from the nozzle outlet, there appears the unevenness due to the small fluorescent intensity. (4) At the medium region, the unevenness at the jet periphery is distinguished and the surroundings enter even at the region of the jet axis. (5) The surroundings existing at the jet tip region diffuse more remarkably. (6) A great deal of soot with very small size are generated at the upstream. (7) The relative soot diameter increases and the relative soot number density decreases due to the coalescence and the condensation as the location goes farther from the nozzle outlet. (8) At the flame tip region, the large soot appears and the relative soot number density increases as the time is passing. (9) The tendency of the rate of the area of medium-size soot to the whole area of soot corresponds to that of the temporal change in the relative mean soot number density.