Pyrazoline derivatives as valuable five-membered heterocyclic compounds could be readily prepared through the intramolecular cyclization of ketazines. In this paper, transition metals (Zn 2+ , Cu 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , and Fe 3+ ) were applied for the catalytic synthesis of 3,5,5-trimethyl-2-pyrazoline, and FeCl 3 showed the best catalytic performance with 95% conversion and 99% selectivity. Substrate screening exhibited that the steric hindrance of ketazines is a key factor on the conversions of R-substituted pyrazolines, following the order of R as methyl (95%) > ethyl (83%) > propyl (68%) > butyl (55%). Moreover, the mechanism of the cyclization of ketazines catalyzed by FeCl 3 is calculated by density functional theory. The energetic profile shows that the cyclization reaction consists of two steps and FeCl 3 can efficiently reduce their barrier energies. This work provides a feasible method for the preparation of pyrazoline derivatives using inexpensive, less toxic, and environmentally friendly FeCl 3 as a catalyst.