The horizontal water cooled diesel engine has a structure including all component parts such as a fuel tank that are necessary to drive engine, and is often a single cylinder engine. It is mounted on many applications such as power tiller and water pump because of high general versatility of installing owing to belt drive. Because the simple structure of single cylinder makes engine reasonable price and low running cost, it is active mainly in Southeast Asia. The market requires this type of engine more high power while a compact structure is also required from the viewpoint of mountability and operability. In other words, “High power density” that is improving the output per body size has been required. We have responded to the demand of “High power density” by increasing output without changing the engine size. In order to keep the engine size, we have been increasing displacement by using our peculiar stroke-up expertise and original bore-up know-how. In addition to those techniques, we introduced analytic technology for early approach to optimal solution. While we had used deep bowl combustion chamber for emphasizing medium and low speed torque, we adopted shallow dish combustion chamber because we shortened the compression height of piston for stroke-up. We utilized combustion analysis so as to approach optimal solution early because we have no base data of shallow dish combustion chamber. In addition, we used also stress analysis to optimize the hardening of crankshaft. As written above, by incorporating analytic technology in addition to conventional development methods, we have been supplying correct size engines speedily in response to requirement of market. In this paper, we introduce the techniques that we adopted in order to realize the high power density.