Abstract: Al-MCM-41 mesoporous molecular sieves with high specific surface area have been hydrothermally synthesized with Shenghua Refinery's FCC decant oil dissolved in toluene as additive, using cationic surfactant of cetyl trimethyl ammonium bromide as template, tetraethyl orthosilicate as silica source and aluminum sulfate as aluminum source in this article. XRD, SEM, nitrogen adsorption and TG-DTA have been used to characterize pore structures of these synthesized samples. Effects of additive on synthetic conditions have been investigated and longer time for blending colloidal solution has been taken. Emphasis has been put on changes of structure properties such as crystallization, crystal cell parameter, BET surface area, average pore diameter and pore volume with addition amount of FCC decant oil under conditions that the ratios of toluene to FCC decant oil are 1∶1 and 2∶1 respectively. Explanations of these varieties have been supported from synthetic mechanism. Results of N2 adsorption reveal that BET surface area and pore volume of molecular sieves synthesized are respectively up to 1163.7m2·g－1 and 134cm3·g－1, and average pore diameter is 4.34 nm. TG-DTA profiles show that for the molecular sieves added by FCC decant oil there existed two clear exothermic phases, which are respectively resulted from the removals of surfactant and FCC decant oil from pores. In addition, DTA profile shows that exothermic peak for removal of the template of the sample added by FCC decant oil is inclined to shift to higher temperature, which explains that decomposition temperature of template molecules bonded with Si-O-Al group is increased with addition of SC- SEM photos show that crystal particles become smaller, more disperse and still look like the shape of bars.