A series of N,N,N-tridentate iron (II) complexes bearing 2-imino-1,10-phenanthrolines, [2-{(2,6-R1,R3-4-R2-C6H2)NC(R)}-1,10-phen]FeCl2, was synthesized and characterized by IR spectroscopy and elemental analysis. One of these complexes (18b) was determined by single-crystal X-ray crystallography for its unambiguous structure. These iron(II) complexes were found to exhibit remarkable activities for ethylene oligomerization in the presence of MAO or MMAO cocatalyst affording α-olefins with high selectivity and the composition of oligomers followed the Schluz–Flory distribution. The effects of substituents on imino fragment on the activity were explored in detail; complexes with mono substituent at the ortho-position of the phenyl ring showed high activities with low K values, suggesting that fine tuning of the steric bulk of substituents on the imino fragment directly affects both the activity and the selectivity.