We conducted a field evaluation using qualitative and quantitative methods to assess freeze prevention of vaccines transported and stored in a recently developed, World Health Organization-prequalified freeze-preventive cold box (FPCB) as compared to currently used standard cold boxes (SCBs). The study assessed the FPCB's practical use, health worker acceptance, health system fit (including cost considerations), and challenges faced by health workers in variable conditions and geographical settings. The evaluation took place in five health facilities across hilly and plains districts of Nepal in two phases: Phase 1 involved FPCBs in simulated use alongside SCBs. In Phase 2, actual vaccines were used in the FPCBs. The study gathered quantitative data from logbooks and electronic temperature monitors placed inside and outside the cold boxes. Qualitative data were collected from health workers, cold chain personnel, and immunization program managers involved in the vaccine cold chain at multiple levels. No damage, durability issues, or freezing incidents were observed when using FPCBs, but two incidents of freezing occurred when using SCBs. FPCBs also took longer to cool down than SCBs. Participants mostly found the FPCB to be safe and user friendly for vaccine transportation and short-term storage. Advantages of the FPCB as compared to the SCB include its ability to minimize vaccine wastage, to keep freeze-sensitive vaccines safe (the average value of freeze-sensitive vaccines transported per shipment was $1,704), and to ease preparation through elimination of the need to condition ice packs. Procurement price ranges for FPCBs overlap those for SCBs. Disadvantages of the FPCB include its greater size and weight, which require more personnel and vehicles during transportation. This suggests that lighter and smaller FPCBs would be more effective and acceptable for the Nepal immunization program and other, similar immunization programs conducted globally.