This study aimed at a better understanding of the transient nature of axisymmetric gas jets and sprays under diesel conditions. To evaluate the performances of the Reynolds-averaged Navier-Stokes (RANS) and the large eddy simulation (LES) modeling approaches, a comparative computational study using both methodologies has been performed for the same computational cases with the same numerical algorithm and spatial resolution. The results show that LES is capable of capturing the unsteady features of transient gas jets and sprays such as flow vortical structures, whereas RANS is incapable of resolving these features. Furthermore, the excessive dissipation in the RANS simulation can lead to underprediction of the jet penetration, while LES can better represent the jet mixing and entrainment, which offers a promising tool for numerical prediction of transient jets and sprays.