Abstract: The flow fields of subsonic free jets, co-axial free jets and overcritical free jets have been researched experimentally and by using the LDA. The measured typical results are shown in figures 2,3,4, 5 and 6. The turbulent jets involve with a wide range of velocity arid turbulence > and involve with the influence of shock waves. the phenomena are complex. For obtaining the good measurement results, the scattering particles must be seeded correctly at first. The flow rate of seeding is dependent on the magnitude of the measured velocity and the field position which will be measured. Then, the optical frequency shift must be used, though in the free jets there is not recirculating zone. At overcritical status, the velocity values in the jet core and behind the oblique shock front can be measured correctly by LDA. But the LDA is lack of the resolution to decide on the position of shock front, as shown in figure 6.