In this letter, we study the outage performance of non-orthogonal multiple access (NOMA) with fixed power allocation over Nakagami-m fading channels, where the fading parameters of NOMA users are different. In this scenario, new closed-form expressions are derived for the outage behavior of individual users and the system, respectively. Next, the diversity orders are obtained under high signal-to-noise ratio condition. The derived analytical expressions are exact and unprecedented in the earlier literature. Finally, simulations are conducted to confirm the validity of the analysis and show the outage performance of NOMA under different fading parameters of Nakagami-m fading channels.