A series of neutrally charged Ni(II) phenoxyiminato catalysts with fluorine atoms at different positions on the N-terphenyl motif are synthesized, and their abilities to polymerize ethylene are compared. At 25 degrees C, the ortho-fluorinated Ni-5F, Ni-3F', and Ni-2F achieve significantly higher polymerization activities than Ni-3F and Ni-OF. In addition, branch density and molecular weight of the obtained polyethylenes vary gradually in the order of Ni-5F, Ni-3F, Ni-3F', Ni-2F, and Ni-OF. Based on the X-ray crystal structure and F-19 NMR spectra, the ortho fluorine atoms are found to make terphenyl groups more rigid and bulky. Theoretical calculations suggest that the increased steric bulk of terphenyl motif leads to an increase in the ground state energy of the resting state species relative to the migratory insertion transition state, and consequently, lowered migratory insertion barriers are expected in Ni-5F, Ni-3F', and Ni-2F. On the other hand, the weak hydrogen bonding between the ortho fluorine atoms and coordinated ethylene in insertion transition state is also proposed in favor of insertion. Similar to previous reports, polyethylene microstructure was mainly related to electronic effects of fluorine atoms.