This paper presents a parametric study on the interaction of twin circular synthetic jets (SJs) that are in line with a crossflow over a flat plate. The resulting vortex structures under differ- ent actuation and flow conditions are investigated using stereoscopic color dye visualization in a water tunnel. The influence of four independent non-dimensional parameters, i.e., the Reynolds number ReL, Strouhal number St, velocity ratio V R, and phase difference ?f, on the behavior of the twin SJs is studied. It is found that the increase of Reynolds number causes the SJ-induced vortex structures more turbulent, making the twin SJ interaction less organized; The increase of velocity ratio pushes the occurrence of interaction further away from the wall, and makes the resulting vortex structures more sustainable; The Strouhal number has no obvious influence on the interaction; And three types of vortex structures are observed under different phase differences: one combined vortex, two completely separated vortices, and partially interacting vortex structures. Based on this parametric study, a simple model is proposed to predict the resulting vortex pattern for the twin SJ interaction.