Abstract The favorite adsorption sites of ethylene (C2H4) and acetylide (CCH) on the Ni(110) surface are investigated by the theoretical calculation of the pseudo-potential plane-wave method. At low coverage the ethylene molecule occupies approximately the midway between the short-bridge and atop sites. The C-C axes of molecules are almost parallel to the ridges of the Ni atoms. The tilt angle of C-C axis relative to the Ni surface is about 12°. The C—C bond length is 0.147nm. At high coverage(0.5ML), ethylene molecules form a c(2×4) phase. The local adsorption sites of two ethylene molecules in a surface unit mesh are found to be quite similar to that at low coverage, but the C—C bond lengths are shorter than that in the case of the low coverage. The adsorption site of cetylide, decomposed from ethylene, is similar to that of the ethylene molecule on the surface, but there is a small displacement towards the atop site, the C—C bond length is 0.131nm, and the hydrogen atom bonds with the carbon a an atop site, the tilt angle of C-H axis relative to the Ni surface is about 45°.