The circumstances of medium speed diesel engine have been severe to keep up with the environmental problem such as global warming or IMO regulation. To adapt the market trend, the authors developed a series of“Neo”medium speed diesel engines, model 22HLX, 28HLX and 34HLX. This paper describes design features, development processes and principal inspection results of these engines. On the development, extensive cost analysis of existing engines was made to realize extremely low cost/output rate. Performance simulation, FEM analysis, 3D module planning and component tests were also carried out. As the result, the engines having high reliability and friendly handling with advantageous performance were developed. These engines achieved high output with increasing power rate (24.5MPa⋅m/s) as the product of high brake mean effective pressure (2.45MPa) and increased mean piston speed (10m/s) . High thermal efficiency was achieved by prolonged piston stroke, soft-Miller-cycle and high performance radial turbocharger. Lightweight mono-block piston and compound oil-groove big-end bearing, for example, were the key components to realize such high output engines. Concurrent engineering and virtual factory production were applied by means of unity administration of allied factories. First productions were successfully launched from 1998 to 1999, and the longest running hours reached over 18000 hours in October of 2001. The FOC, LOC and exhaust gas temperature was at low level as expected. At the first principal inspection, the piston top land and piston ring grooves were far clean rather than previous engines. Piston and cylinder liner condition was perfect.