Although Steam injection is a potentially effective and efficient improved heavy oil recovery method, it can be extremely challenging process in naturally fractured heavy oil reservoirs. Communication between matrix and fracture provides a coupling of the two distinct and contrasting systems which results in a highly non-linear problem, if added complexity changes in fluid properties due to the thermal effects of steam then it gets even more complicated. Accurate reservoir and fluid characterization as well as steam injection operation parameters and dynamics are must for reliable modeling of such complex process. It this study the steam injection including steam flooding and Cyclic Steam Stimulation (CSS) are studied and compared in detail as potential development scenario of a highly fracture heavy oil reservoir in Iran. The field is giant structure like a symmetrical anticline with 90 km length and 16 km width in the surface and also 60 km length and 10 km width on 1000 mss depth with about 3.6 billion barrels of initial oil in place. Initial pressure is about 927 psi in 1700 ft below the sea level. The oil gravity is 8-12 API with about 2700 cp viscosity at surface condition. In this work, a thermal dual porosity model for the reservoir structural, stratigraphic, petrologic and petrophysical description of the reservoir, geometry, rock types/ properties, layering, porosity types, fracture orientation and density, faults, etc are used. The steam interaction and the subsequent property changes with the reservoir fluid and rock are studied and incorporated in the 3D reservoir model. The result shows that steam flooding could improve the recovery factor from almost zero to about 12 while CSS will give about 37 of recovery factor in the studied sector model which makes it more attractive method as development scenario. Furthermore the results illustrated that the injection strategies, well spacing, well type, pattern type and size are among the important parameters for designing the steam injection. Additionally, the effect of other parameters such as injected steam quality, oil-water capillary pressure in matrix blocks, steam injection and oil production rate have been studied for both steam flooding and CSS process. © Copyright 2011, International Petroleum Technology Conference