In this research the creep behavior of heat resistant steel HP40, produced in Akhgar Steel Foundries of Isfahan, was studied. These alloys are widely used in construction of steel tube of reformer furnaces in petrochemical industries and in iron production industries to produce reducing gases. These steel tubes are produced by horizontal centrifugal casting method. Due to environmental conditions of reformer furnace (i.e., temperature 700 up to 1100?, pressure 1-5 MPa) the process of creep in the tubes is very active. So, the alloy used must be of high creep strength. One way to increase the creep resistance of heat resistant alloys is to add titanium as a good carbide-stabilizing element to the composition of the alloy. Using horizontal centrifugal casting method, in this study four samples with zero values, 0.06, 0.21 and 0.23wt% of Ti were produced. To gain knowledge of mechanical behaviors of the samples tensile, hardness and creep tests were performed. Microstructures of the samples were examined by Optical and electron scanning microscopy. It was found that by increasing the amount of Ti in the chemical composition of the alloy, the distance between secondary dendritic arms and size of grains in tube cross-sectional area reduced. Titanium reduced the amount of continuous grain boundary carbides, also caused producing finer carbides and well deformation of the skeletal modes to the spherical shape Chromium carbides. The 100- hour creep tests were designed at 40MPa constant stress and temperature of 900?. The sample with 0.06%wt. of Titanium was the only specimen that resisted up to 100 hours with of steady-state creep rate of 6×10E-6. Longitudinal sections of the fractured specimens after the creep tests showed that under stress, cracks nucleates at the chromium carbides-matrix interface and grow in the area. By increasing titanium, secondary-formed carbide precipitated during creep tests were of niobium-riched and titanium-riched precipitates. Linear and point analysis of phases showed that in all the samples a G-destructive phase with a composition of Ni 16 Nb 17 Si 6 was formed during the creep test. But its presence was much less in titanium-containing samples in comparison with titanium- free ones. So, there appears that the presence of titanium hinder the transformation of the composed niobium-titanium carbides to the G-phase. Based on data obtained from the creep tests and using the Larson Miller parameter, approximate life of tubes at service condition i.e., 22.5MPa stress and 900? temperature, was estimated to be more than 10 years. Keywords: Creep, Niobium – Titanium Carbide, Reformer, Heat Resistant Alloy HP40, Horizontal Centrifugal Casting