Porous silica SBA-15 was synthesized using triblock copolymer P123 as structure template and tetraethyl orthosilicate (TEOS, C8H20O4Si) as silicon source. The pore structure was adjusted by using swelling agent 1,3,5-trimethylbenzene (TMB). Small angle X-ray scattering (SAXS) and high resolution transmission electron microscopy (HRTEM) indicate that the pore morphology changes in three stages with increasing TMB/P123 ratio. In the first stage (TMB/P123 < 0.2), the pore type is a typical cylindrical hole. When TMB/P123 is 0.2-0.5, the pore morphology is a curved cylinder. In the final stage when TMB/P123 ratio is higher than 0.5, the ordered mesoporous silica is converted into mesoporous silica foam. The average pore size estimated from N-2 adsorption measurements increases from 6.5 nm to 17.2 nm with increasing TMB/P123 ratio from 0 to 2. Positron lifetime measurements indicates two long lifetime components tau(3) and tau(4), which are due to o-Ps annihilation in micropores and mesopores, respectively. It was found that the mesopores are open pores and the micropores are closed pores. With the increase of TMB ratio, the number and size of micropores show nearly no change, but the mesopore size increases gradually with increasing TMB/P123 ratio up to 0.5, which are in agreement with the results of the N-2 adsorption/desorption measurements. However after the pores evolves into mesoporous foam, the pore size obtained from o-Ps lifetime is less than that of N-2 adsorption/desorption measurements.