The electronic states of tetracene adsorbed on Ru (1010) surface as well as their adsorption sites and directions has been studied by using ultraviolet photoelectron spectroscopy (UPS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and scanning tunneling microscopy (STM). The UPS measurements show that the six peaks from the tetracene are 2.1, 3.5, 4.8, 6.0, 7.1 and 9.2 eV below the Fermi level. The ARUPS measurements suggest that the plane of tetracene molecule is parallel to the Ru(1010) surface. The STM image shows that the tetracene molecule is adsorbed on Ru (1010) surface with its longer axis lying in the direction of either ［0001］ or ［1210］. The adsorption structure was confirmed with an ab initio calculation based on density functional theory (DFT). The short-bridge site and the hollow site are the most favorable sites when the molecule has its longer axis oriewted along ［0001］ and ［1210］, respectively.