Calculation of electronic structures and magnetic properties of Mg （or Zn＼Si＼O） and Mn co-doped GaN were carried out by means of first-principle plane-wave pesudopotential （PWP） based on density functional theory. Estimation of Curie temperature was achieved by using Heisenberg model in the mean-field approximation and Zener theory，respectively. The spin polarized impurity bands of deep energy levels are found for several co-doped systems, which are half metallic and suitable for spin injectors. Compared with GaN：Mn，p-type co-doped （GaN：Mn-Mg＼Zn） systems exhibit more stable ferromagnetic state and a significant increase in TC. Nevertheless，n-type co-doped （GaN：Mn-Si＼O） systems fail to increase the TC and stability of ferromagnetic state.