The paper presents the flow mechanism of a reattachment process over a backward facing step, which is controlled by vortex generator jets (VGJs). The injection of VGJ upstream of the step introduces the co-rotating longitudinal vortices in a separating shear layer. VGJ produces a primary and secondary vortex. The detailed structure of the longitudinal vortices is clarified, and the effect of the vortices on the separating shear layer downstream of the step is observed. The influence of the velocity ratio and the injection pitch on the flow mechanism is also discussed.