An attempt is carried out to make the knots in multifilament yarns with conventional interlacer that is controlled to investigate the relationship between the interlacing results and processing conditions. In order to analyze the multifilament movements in interlacer, images of multifilament yams in real time using high speed camera are captured. The physical properties of interlaced yarn are measured to evaluate the influences of two processing conditions, namely, air pressure and yarn tension. Interlacing properties that are caused by the changes of process conditions based on the captured images are analyzed. The mechanism of interlacing in interlacer with the captured images of consecutive multifilament movements were inferred. The movements are classified into several stages according to the status of knot formation which is mainly dependent on the process conditions. Experimental results can be utilized in producing the optimum interlaced yarns for end-use application.