Abstract: This paper reports the experimental investigation of the turbulence characteristics of the free air jets issuing from rectangular orifices with different aspect ratios (AR=1~15). The velocity was measured by a hot-wire anemometer with the exiting Reynolds number (Re) being 15,000 for all jets, based on the equivalent diameter of the rectangular orifices. This paper mainly analyzes the evolution of centerline average velocity, frequency spectrum, turbulence scale along the centerline of rectangular jets. The results show that the centerline mean velocity decays faster and the turbulence intensity grows higher as AR increases. With jet flows developing downstream (x/D_e>30), the turbulence energy spectrum, probability density function and turbulence length scales of rectangular jets approach to those of circular jets. The principle for this phenomenon is that the momentum from the jet always spreads into still surroundings in more efficient way.