The velocity field in an opposed jet mixer was studied by using Phase Doppler Anemometry (PDA). In this study, the effect of operating conditions such as nozzle separations and inner nozzle diameters was investigated to study the changes in characteristic chaotic parameters (correlation dimension, Kolmogorov entropy, and largest Lyapunov exponent). The velocity field possessed chaotic characteristics, in which the value of the correlation dimension ranged from 2.34 to 7.52. The characteristic chaotic parameters increased and then decreased as the nozzle distance increased. This reflected the micro-mixing efficiency of the mixer. The efficiency of the mixer improved as the values of the chaotic characteristic parameters increased. Thus, the optimal conditions of microcosmic mixture were obtained. Therefore, a suitable nozzle distance, or decreasing nozzle diameter, is advantageous for the improvement of a mixer's micro-mixing efficiency.