A nitrogen double expander cycle has been widely used for liquefaction of natural gas in LNG-FPSO (Floating Production, Storage, and Offloading). An aluminum plate-fin heat exchanger (ALPHE) is usually adopted in the liquefaction cycle. In general, the ALPHE has a very large heat transfer surface area per unit volume. This surface area consists of primary and secondary (finned) surfaces. Even taking into account fin efficiency of the secondary surface, the effective surface area per unit volume can be typically five times greater than that of a shell-and-tube heat exchanger. Various types of fin are available in ALPHE and the fin type should be selected properly to optimize the performance. For example, serrated, wavy and perforated fin are particularly suitable for gas streams. The selection and design of the layer arrangement and effective length of each stream are very important design parameters for ALPHE. In this paper, to optimize the design of ALPHE, the effects of the design parameters on the performance of ALPHE were studied using a simulation method. The properties of nitrogen and natural gas were calculated from proper equations of state. Because the performance of ALPHE is mainly influenced by the fin type, fin frequency, fin height, fin thickness, and layer arrangement, the effects of the geometric design parameters on the performance of ALPHE were studied, and the optimum design conditions were suggested in this paper.