In this work, the interference characteristics of two parallel underexpanded jets issuing from straight circular pipes with different diameters were studied. Further, the influence of the distance between the central axes of the two pipes on the interference of jets was investigated. The inner diameters of the pipes used were 4.1 and 6.1 mm, and 6.1 and 9.1 mm, respectively. Flow visualization and measurement of velocity distributions were conducted ; thus, the mechanism of pseudo-shock waves and equivelocity diagrams within a vertical section to the jets axes were clarified. As a result, it was found that the longitudinal vibration of flow was controlled by the difference between the cell lengths of the two jets. Thus, the supersonic flow was present in a considerably farther downstream range than the case with two parallel jets with equivalent inner diameter.