In the present paper, the effect of the collar plate on the static capacity of circular hollow section (CHS) X-joints under brace compression is experimentally and numerically investigated. For this aim, three X-joints reinforced with collar plate, with different values of β, and three corresponding unreinforced X-joints were tested. All of the details and results of the experimental tests are presented. The static behavior of the collar plate reinforced and un-reinforced X-joints is then evaluated from the load-displacement curves and deformed shapes. The compression between the results of reinforced and unreinforced X-joints shows that the collar plate can increase both of the initial stiffness and ultimate strength. Also, these beneficial effects of the collar plate are significant in all of the joints with small, intermediate, and big values of the brace-to-chord diameter ratio (β = 0.25, 0.54, 0.73). Moreover, the compression between deformed configurations shows that deformed shape of the reinforced specimens in the near joint intersection is more uniform than the deformed shape of the corresponding unreinforced specimens. Finally, finite element (FE) model is proposed. Results show that the present FE model can accurately predict the load-displacement curves and failure modes of the unreinforced and collar plate reinforced X-joints.