The Si doping mechanism is systematically investigated in dilute nitride GaAsN grown by radio frequency plasma assisted molecular beam epitaxy (RF-MBE). We change growth temperature, Si impurity concentration ([Si]), and nitrogen composition ([N]). The relationship between Si activation ratio (alpha) and [N] are evaluated using X-ray diffraction (XRD) 2 theta-omega and Hall effect measurement. As [N] in GaAsN increases, alpha drastically decreases, which is ascribed to mechanisms of inactive Si donors such as a cluster of Si and N at an As site ((Si-N)(As)). We find that the main factor of inactive Si donors in GaAsN depends on both [Si] and [N].