Structural characteristics of air-jet spun yarns were studied through the three dimensional configuration of the tracer fibre. MJS (Murata Jet Spinner) was chosen as an air-jet spinner. 100 % polyester air-jet yarns were spun from the drawn sliver which included about 1 % tracer fibre under various spinning conditions. Spinning conditions are spinning speeds, ratios of the pressure at the first nozzle to that at the second nozzle, and linear densities of yarns. By the way, conventional ring yarns and open-end rotor yarns were spun from the same sliver at the linear densities of Ne 20, 30 and 40. In all sample yarns, the configuration of the tracer fibre was observed and photographed by a polarizing microscope. The three dimensional position of the tracer fibre was calculated from the photograph. Further the distribution of the fibre orientation and that of the fibre packing density were obtained from the three dimensional position and were analyzed by a statistical-analysis-system package. The orientation of fibres was described by polar angles and azimuthal angles. It was shown that the polar angles had gamma density but the azimuthal angles were distributed randomly. The distributions of the polar angles of the fibres in ring yarns or open-end rotor yarns do not vary according to the change of radial position. But in case of MJS yarns, there are many straight fibres in the core, which are arallel to the yarn axis. Numbers of fasciate fibres are proportional to the thicknesses of MJS yarns. The head and tail parts of a fibre are located in the outerwall of the yarn. The mean values of polar angles of fibres in MJS yarns increase with increase of the spinning speed or the pressure at the first nozzle.