Hermal cracking of a heavy liquid hydrocarbon was performed in a laboratory scale tubular reactor. Central Composite Design (CCD), was used as an experimental design method. The design variables were Coil Outlet Temperature (COT), feed flow and rate steam ratio.Maximum yield of ethylene was 30.37 wt% at COT, residence time and steam ratio of 869 oC, 0.208 s and 1.22 g/g, respectively. Maximum yield of propylene was 15.37 wt% at COT, residence time and steam ratio of 825 oC, 0.147 s and 0.95 g/g, respectively. A mechanistic model based on free radical chain reactions was developed using experimental results. Developed reaction network contains 148 reactions for 43 species. Finally, the experimental data were compared with model results.Scatter diagrams showed good agreement between model and experimental data.