Steam-Assisted Gravity Drainage is an effective approach for recovering heavy oil and bitumen and its essence idea is to introduce steam heat into cold reservoir and reduce oil viscosity. In a typical SAGD process, two horizontal wells were drilled inside the target formation, one is put on the top as steam injection well and another is usually put on the bottom of target formation as oil production well. During SAGD process, a large amount of high pressure high temperature steam is injected into reservoir which occupies a big part of the whole SAGD project cost. The steam pressure and quality decreases during the steam flow inside vertical wellbore because the steam loses its heat through wellbore system to formation due to temperature gradient. And in horizontal wellbore, steam even flow into formation through slotted liner which takes away energy directly. For a SAGD project, the steam pressure and steam quality insides steam injection well are quite important parameters. A high enough quality can offer enough energy for steam chamber to develop and steam pressure will influence the oil production rate, steam trap control as well as ultimate oil recovery. In order to control the cost of SAGD production and offer evidence for steam injection, oil production strategy, the knowledge of steam properties (pressure, temperature, quality) along both vertical and horizontal wellbore are needed. The steam flow inside wellbore is a two phase (dry steam and hot water) flow and the determination of phase void fraction is critical in predicting the pressure loss and heat transfer. The major difference between steam flow inside horizontal wellbore and vertical wellbore is the existence of wall outflow in horizontal wellbore part. This wellbore outflow has a significant effect on wellbore friction as well as flow pattern transition inside wellbore which make it difficult to describe flow pattern in horizontal wellbore by former technology. A model describe steam flow inside wellbore during conventional Steam Assisted Gravity Drainage stage (after the steam chamber has achieved full height and lateral growth becomes the dominant mechanism for recovery) was built and solved in this thesis. A flow pattern independent drift flux model based void fraction correlation was introduced in this thesis in order to overcome the uncertainty problem in determining flow pattern and making is possible to describe steam flow in both vertical wellbore and horizontal wellbore in an unified way. A modified Reis’s drainage model was used in this thesis which combined steam flow inside wellbore and steam chamber development inside formation. The steam properties (pressure, quality) distribution along wellbore trajectory were calculated, the effects of basic steam injection parameters (pressure, quality and mass flow rate) were analysed in this thesis. These steam property profiles along wellbore trajectory actually build correlations between wellbore flow and oil production, and will improve the understanding in steam injection strategy adjustment as well as oil production dynamic monitoring