The nanostrutured mesoporous silica materials MCM-41 with different pore sizes were synthesized by adding 1,3,5-trimethylbenzene (TMB) as assistant agent in the reactant mixture. The obtained samples were investigated by XRD, HRTEM, FTIR and N2 adsorptions. It is shown that the pore sizes, ordered degree and specific surface area increase effectively with the incremental addition of TMB in an appropriate range, but the (100) peak intensity of XRD patt becomes weakened and the ordered degree decreases when the additive quantity continued to increase. With the addition of TMB in an appropriate range, the effective surfactant ion pair packing parameter, g=V/a0l, is mediated in the range of 1/3-1/2 with the increment of V and l, indicating that mesoporous silica is a well-ordered hexagonal structure. However, when the addition of TMB increases, g value becomes larger, which leads to a less long-range ordered structure for the uneven condissolution of TMB.