Experimental investigations have been carried out for the breakup process of an annular water jet exposed to an inner air stream by videographic and photographic techniques. Three flow regimes for the jet breakup process have been identified-i.e., bubble formation, annular jet formation, and atomization. Within the bubbleformation regime, the jet breakup characteristics measured from the photographs taken under various liquid and gas velocities indicate that nearly uniform bubbles were observed for various air-to-water velocity ratios; both the jet breakup length and the wavelength for bubble formation decreased as the air-to-water velocity ratio was increased. The measurements were compared with the predictions by linear instability analysis, and fair agreement was obtained.