Superconductivity and the normal-state properties of NbN films can be tuned in a wide range, supplying a suitable platform to investigate the systematical evolution of the superconducting performances. Herein, we report the upper critical field of NbN films in both the vertical (B ⊥ film) and parallel (B ∥ film) orientations over a wide temperature range. Eight samples with the superconducting critical temperature T c ranging from 2.5 K to 9.8 K are studied. It is found that the magnitudes of the upper critical field in the parallel direction (B c 2 ∥) exceeds the paramagnetic limiting field B p in the zero-temperature limit, while the perpendicular one (B c 2 ⊥) is closely approaching B p. The temperature dependent B c 2 ⊥ can be described by the extended Werthamer–Helfand–Hohenberg (WHH) model considering the Pauli spin paramagnetism. Meanwhile, the B c 2 ∥ − T data shows the feature of two-dimensional superconductivity in the temperature range near T c. Our results show that the Pauli paramagnetic effect is greatly enhanced with the decrease of T c. The evolution of the obtained Maki parameter with other parameters are discussed to reveal the characteristics of the Pauli paramagnetic effect in this system. • B c2 || exceeds the Pauli-limiting field B p , while B c2 ${\perp }$ is closely approaching B p. • B c2 ${\perp }$ - T curves can be described by WHH model consideringPauli paramagnetic effect. • The Pauli paramagnetic effect is greatly enhanced with the decrease of T c. [ABSTRACT FROM AUTHOR]