The N2 dissociation yield has been measured in a flowing post-discharge containing H2 between 0 to 5%. The N atom density has been determined in the fast post-discharge (Δt = 5 × 10−2 s) electric field has been measured by electrostatic probes as a function of H2 relative concentration. It is demonstrated that the H2 produces a significant increase in the N-atom concentration measured in the post-discharge. For our experimental conditions it was determined an increase up to 3 times in comparison to pure nitrogen discharge. The maximum yield of N-atoms was obtained for a H2 relative concentration of about 2%. This result cannot only be explained by the decrease of N atoms surface losses. It is demonstrated that the N2 dissociation increase is also the result of an increase of the discharge reduced electric field.