A large size multistream plate fin heat exchanger (MSPFHE) is developed for an in-house-developed modified Claude cycle-based helium liquefier. The development of this heat exchanger, including thermal sizing and mechanical design, is described in this article. A pressure drop analysis is also carried out to compute the pressure drops across various components of the MSPFHE. The MSPFHE, after fabrication, pressure and leak testing, is integrated with the rest of the process equipment and piping of the developed helium liquefier cold box. Experiments are conducted to evaluate the performance of the developed MSPFHE under different modes of operations of the helium liquefier. Experimental results are compared with the computed predictions based on the two-dimensional numerical model as well as the commercial software Aspen MUSE and reported in this article. Good agreement between the computed and measured performances is observed during the field testing of the MSPFHE.