Air-interlacing of filament yarns is a method of improving yarn properties, preventing faults during production processes and abbreviating or simplifying yarn production methods. Recently, combination with various yarn manufacturing process and application possibilities of air interlaced composite yarns have been actively investigated. As methods evaluating the quality of interlaced yarn are subjective and inaccurate, a new analysis system for more objective evaluation is needed. In order to analyze interlaced yarns on an on-line system, we examined the possibility of measuring structural properties of interlaced yarn using a photo-electric sensor. The diameters of interlaced yarn were measured by means of an photo-electric sensor, and the effects of measuring length and measuring speed on various evaluation values were studied . Experimental results revealed that new technique could provide objective information of interlaced yarn characteristics, allowing real-time analysis.