The fuel cell is known as environment friendly power generation plant. Among various types of fuel cells, phosphoric acid fuel cell (PAFC) is the only one commercial. In this paper, our process for the development of the fuel cell stack that is the most important part of PAFC is shown. Regarding the reliability, we observed the rapid break of several cells in the fuel cell stack, which is consisted of the new type cells. We found new phenomenon of phosphoric acid movement in the cell. As for durability, we improved air electrode catalyst. We also improved the catalyst layer consisting of catalyst and PTFE with consideration of the zeta potential of the catalyst and PTFE. This is to mix the catalyst and PTFE well. As the result, PAFC has enough durability and reliability for practical use, and we are expanding PAFC business.