In plate fin heat exchangers, fins of special shapes on the plates generate high turbulent flow in the flow channels formed by the plates, which enables achievement of high heat transfer coefficients. The present work uses a conjugated heat transfer approach to evaluate numerically the heat transfer improvement and pressure drop in plate fin heat exchangers due to zigzag (type B) and inner zigzag-outer zigzag (type C) rectangular fins 4 mm high. Rectangular fins are mounted on a flat plate channel (type A). The numerical computations are performed by solving a steady three-dimensional Navier-Stokes equation and an energy equation using the Fluent software program. Air is taken as the working fluid. The study is carried out at Re = 400, the inlet temperatures and velocities of the cold and hot air fixed as 300 K, 600 K, 1.338 m/s, and 0.69 m/s, respectively. Variation of the Colburn j factor with Re design data is introduced by using the Fluent software program. This study exhibits new fin geometries for plate fin heat exchangers; the fin types used in this study to improve the heat transfer rate have not been investigated yet in the literature. The results show that the heat transfer is increased by about 9% at the exit of channel with the type B fin as compared with a channel without fins for counter flow. The heat transfer enhancement with zigzag and inner zigzag-outer zigzag fin types for Reynolds numbers of 300, 600, and 900 and varying fin heights and temperature distributions of fluids on the top and bottom surface of the channel are investigated for parallel and counter flows.