In 2006, Nissan began limited leasing of the X-TRAIL FCV equipped with their in-house developed Fuel Cell (FC) stack. Since then, the FC stack has been improved in durability, cold start-up capability, cost and size with the aim of promoting full-scale commercialization of FCVs. However, reduction of cost and size has remained a significant challenge because limited mass transport through the membrane electrode assembly (MEA) has made it difficult to increase the rated current density of the FC. Furthermore, it has been difficult to reduce the variety of FC stack components due to the complex stack configuration. In this study, improvements have been achieved mainly by adopting an advanced MEA to overcome these difficulties. First, the adoption of a new MEA and separators has improved mass transport through the MEA for increased rated current density. Second, an integrated molded frame (IMF) has been adopted as the MEA support. One advantage of the IMF is its simplified structure and production process, reducing the MEA cost. Another advantage of the IMF is that its flexibility for forming structural shapes which has been utilized to build insulating side walls (ISWs) around the FC stack. Third, the FC stack structure has been simplified by elimination of a FC stack enclosure and adoption of a single-row configuration which have been enabled by ISWs. As a result, the power density of the new FC stack has been increased to 2.5 kW/L, which is 30% higher than that of the previous 2008 model. It is estimated that the mass production cost of the new FC stack can be halved to nearly meet the target of the DOE Hydrogen Program. With further improvements such as higher current density and lower catalyst loading, FCVs will be compact and affordable enough for ordinary consumers at the full-scale commercial stage.