Due to increasing the heat transfer surface area and high providing capillary pressure with high permeability, porous structures play a key role in improving the performance of two phase heat transfer devices such as heat pipes. New porous structures (bi-porous structures), have two distinct size distribution of pores of which the small pores provide the capillary pressure required for delivering liquid to the surface and large pores help vapor escape from the surface through increasing its permeability. The main goal is to gain a deeper understanding of the evaporator section of heat pipes and comparison between the performances of two sample biporous structures. Towards this goal first the Kovalev modeling technique is applied to determine the possibility of each phase’s existence in pores of different sizes throughout the computational domain. One dimensional heat transfer in a bi-porous wick is investigated. Inside the domain the conservation equations are solved for each phase and the results such as heat flux versus wall superheat are presented. Thermo-physical properties of the fluid and the matrix like the fluid properties, phase saturation and permeability and the conduction heat transfer coefficient are calculated from the geometry of the matrix and experimental relationships.