The transitional states of surface structure of TiO2(110) from p(1 x 1) to p(1 x 2) on low reduced to heavily reduced surfaces were studied using, Impact Collision Ion Scattering Spectroscopy (ICISS). We found that (i) bridging oxygen rows were formed on low reduced surface at anneal temperature of 650 degrees C for 2.5 h, (ii) Ti2O unit rows were formed between the bridging oxygen rows on low reduced surface at anneal temperature of 930 degrees C for 100 min, (iii) five-fold titanium atom rows on the surface and titanium atom rows of the Ti2O rows are imperfectly covered with oxygen atoms on heavily reduced surface at anneal temperature of 600 degrees C for 1 h and followed by 670 degrees C annealing for 5 min after ion sputtering, and (iv) Ti2O3 unit rows proposed by Onishi and Iwasawa [H. Onishi and Y. Iwasawa, Surf. Sci. Lett., 313, 1994, L783] were completed by covering with oxygen atoms on all of the titanium atom rows on the Ti2O rows on heavily reduced surface at higher anneal temperature of 1050 degrees C for 40 min after ion sputtering. It is noticeable that the p(1 x 2) surface structures on low reduced surface is different from that of heavily reduced surface, viz., each has Ti2O unit rows and Ti2O3 unit rows, respectively. The result contradicts "missing unit model" proposed by Bennett et al. [R.A. Bennett, P. Stone, N.J. Price, M. Bowker, Phys. Rev. Lett., 82, 1999, 3831].