The catalytic activity of trimethyl phosphite modified HZSM-5 zeolite and un-modified HZSM-5 zeolite treated with 100% steam at 673,773,873,973 and 1073K, respectively, were investigated using heptane cracking as a probe reaction. The results showed that the heptane conversion of both trimethyl phosphite treated samples and un-phosphated samples decreased with an increase in treating temperature, but trimethyl phosphite modified samples showed higher activity in comparison with the un-modified samples, which were steam-treated at a higher temperature. The results were firstly elucidated by the model cluster method and computational quantum chemistry method. Full optimization and frequency analysis of all cluster model have been carried out using the Gaussian 94 software-package with the PM 3 semi-empirical method performed on small cluster models. The computational results showed that the dealumination of trimethyl phosphite modified zeolite model cluster was more difficult than that of un-modified zeolite model cluster when they were treated with steam while investigating the heat of reaction. The catalytic activity of trimethyl phosphite modified HZSM-5 zeolite and un-modified HZSM-5 zeolite treated with 100% steam at 673, 773, 873, 973 and 1073K, respectively, were investigated using heptane cracking as a probe reaction. The results showed that the heptane conversion of both trimethyl phosphite treated samples and with un-phosphated samples decreased with an increase in treating temperature, but trimethyl phosphite modified samples showed higher activity in comparison with the un-modified samples, which were steam-treated at a higher temperature. The results were first elucidated by the model cluster method and computational quantum chemistry method. Full optimization and frequency analysis of all cluster model have been carried out using the Gaussian 94 software-package with the PM 3 semi-empirical method performed on small cluster models. The computational results showed that the dealumination of trimethyl phosphite modified zeolite model cluster was more difficult than that o f un-modified zeolite model cluster when they were treated with steam while investigating the heat of reaction.