A terpyridine derivative DPTP [di-(4-methylphenyl)-2,2':6',2"-terpyridine] was conveniently synthesized from 2-bromopyridine via halogen-dance reaction, Kharash coupling and Stille coupling reaction. Then its corresponding ruthenium complex Ru-DPTP [N,N,N-4,4"-di-(4-methylphenyl)-2,2':6',2"-terpyridine-N,N,N-tris(iso-thiocyanato)-ruthenium(Ⅱ) ammonium] was obtained and fully characterized by IR, UV-Vis, ESI MS and elemental analysis. The MLCT absorption band of Ru-DPTP was blue-shifted from 570 to 500 nm upon addition of Hg2+. Among a series of surveyed metal ions, the complex showed a unique recognition to Hg2+, indicating that it can be used as a selective colorimetric sensor for Hg2+.