The propagation velocity modification of μs helium plasma gun (PG) discharge with nitrogen gas admixture in the helium buffer is studied in relation with ionization front imaging and helium-nitrogen plasma features. From 0,1 % to 0,8 % of N2, the ionization front is faster, more extended and with strong luminous intensity, compared to the high purity helium. The velocity has an optimum, for approximately 0,3 % of N2. Above 0,3 %, the velocity is progressively slowing down, and finally gets slower than the pure helium case above 0,9 %. The velocity evolution may be linked with kinetic and energy transfer predominantly on N2+ or N2 according to the increase of N2 %. Consequently, a local reinforcement of electric field in the front of the discharge is suspected for low N2 percentage, involving velocity increase of the discharge.