The residual slag after the refineing of sand iron has several and valuable components, such as SiO2, TiO2, Al2O3, Fe2O3, CaO, MgO, V2O3 etc.. When the slag is in glassy state, it easily dissolves in most mineral acids.In the present paper, the influence of seed on the formation of titanium oxide hydrates by the hydrolysis of titanium sulfate solution which is obtained by the sulfuric acid treatment of the slag and of SO3 on the formation of rutile-type titanium dioxide by heating of the hydrates are studied.The obtained results are as follow;(1) An effective seed in the hydrolysis of titanium sulfate solution in sulfuric acid solution is anatase-type hydrate.(2) The content of SO3 in the hydrates is reduced by the use of anatase-type seed in the hydrolysis.(3) Most of SO3 in the hydrates is removed by the washing with NaOH solution and Na-citrate solution (alkalic with NaOH).(4) In spite of the fact that the formation of oxygen vacancies in the oxygen structure of titanium dioxide accelerates the thermal transition of anatase to rutile, the hydrates containing SO3 convert more easily to rutile in O2 atmosphere than in air and N2 atmosphere. From this experimental result it is suggested that the rapid reduction of SO3 in titanium dioxide in O2 atmosphere promotes the thermal transition of anatase to rutile.(5) In the chromaticity coordinates, the whiteness of rutile-type titanium dioxide which was prepared from the slag in this study is approximately equal to that of a commercial rutile-type titanium dioxide.