The plasma diffusion has been investigated with respect to the atmospheric-pressure plasma jets (APPJ). The ambi-polar diffusion equation is solved in the radial and axial directions. To explain the plasma diffusion process, the plasma diffusion equation is derived with the charged particle motion and the plasma fluid continuity equation1. Supposing the initial plasma density n(t=0, r, z), the plasma flux Γ D =−D a ∇n and the diffusion velocity u D =−D a (∇n)/n are calculated.