Mesoporous silica SBA-15 was synthesized by hydrothermal method in strongly acidic medium (pH<1) and using the tri-block copolymer, poly(ethylene oxide)20 – poly(propylene oxide)70 – poly(ethylene oxide)20 or (PEO20PPO70PEO20) as a structure directing agent. The gel composition was 1.00 TEOS : 1.65x10-2 PEO20 PPO70 PEO20 : 6.95 HCl : 14 H2O. The Al-SBA-15 catalysts with various Si/Al molar ratios were synthesized by reacting SBA-15 with a solution of sodium aluminate for 12 h. The optimum ion exchange condition of Na+ of Al-SBA-15 by H+ was investigated. H-Al-SBA-15 could be obtained by treatment with 0.01 M NH4Cl or 0.01 M HNO3 under reflux at boiling temperature for 24 h. The synthesized catalysts were characterized using X-ray diffraction, inductively coupled plasma-atomic emission, solid state 27Al-NMR, nitrogen adsorption and scanning electron microscopic techniques. The untreated and treated H-Al-SBA-15 catalysts were tested for their catalytic activities in the cracking of polypropylene at atmospheric pressure. The optimum polypropylene cracking condition is at the reaction temperature of 380℃, 10 wt% of catalyst and under N2 flow of 20 cm3/min. All of H-Al-SBA-15 catalysts with Si/Al molar ratios varied in the range of 10-100, exhibit the polypropylene conversion over 96%. The product obtained is mainly in liquid fraction at the yield in the range of 67-73% with the same boiling point range compared to that of standard gasoline. The spent H-al-SBA-15 catalyst can be regenerated by calcination at 550℃ for 5h, and the regenerated catalyst exhibits similar activity and gives the product composition close to that for the fresh one.