We report an in situ TEM investigation of the growth and transformation in nanometer-sized titania powders. The powders were produced through precipitation of titanium tetrachloride under different pH conditions. The initial phase of the produced powders was amorphous or was a mixture of anatase and brookite according to the pH conditions. During calcination, the anatase particles grew and transformed into rutile. The transformation temperature increased with increasing pH value. In situ TEM observations showed that the anatase particles were absorbed into rutile, and then rutile particles grew by coalescence. Furthermore, small pores were observed to form in samples prepared with high pH from the effects of hydroxyl ions and zeta potential. Pore formation increased the surface area. which delayed the transformation and nucleation of rutile. This explains the difference of growth and transformation of titania powders produced under different pH conditions during calcination.