A series novel 1-(3-substituted-5-phenyl-4,5-dihydropyrazol-1-yl)-2-thio-ethanone derivatives as potential telomerase inhibitors were designed and synthesized. The bioassays demonstrated that compounds 4a, 4f, 4j and 7b, 7d occupied high antiproliferative activity against SGC-7901, MGC-803, Bcap-37 and HEPG-2 cell lines. By a modified TRAP assay, some title compounds were tested against telomerase, and compound 4f showed the most potent inhibitory activity with IC₅₀ value at 0.92 ± 0.09 μM. The mechanism of antitumor action indicated that title compounds 4f and 7b could suppress cell proliferation through inducing cell cycle arrest in G0/G1 phase.