•Ir1–Ir4 possess selectivity between human cancer and normal cells.•Ir1–Ir4 enter cancer cells and normal cells via different mechanisms.•Ir4 shown much more potential than Ir2 and Ir3, even though they are isomers.•Treatment of Ir1–Ir4 results in generation of ROS and prominent depletion of Δψm.Four cyclometalated iridium(III) complexes [Ir(dfppy)2(L)]+ (dfppy = 2-(2,4-difluorophenyl)pyridine, L = 6-(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, Ir1; 6-(isoquinolin-1-yl)-1,3,5-triazine-2,4-diamine, Ir2; 6-(quinolin-2-yl)-1,3,5-triazine-2,4-diamine, Ir3; 6-(isoquinolin-3-yl)-1,3,5-triazine-2,4-diamine, Ir4) have been synthesized and characterized. Distinct from cisplatin, Ir1–Ir4 could specifically target mitochondria and induced apoptosis against various cancer cell lines, especially for cisplatin resistant cells. ICP-MS results indicated that Ir1–Ir4 were taken up via different mechanism for cancer cells and normal cells, which resulted in their high selectivity. The structure-activity relationship and signaling pathways were also discussed.