In these few years the systematic studies have made clear the stress distributions on diesel engine crank shafts under static load conditions. However very little is known about the stress conditions in operating engines, mainly because of difficulties of measurment. Authors carried out the dynamic stress measurments in a four-stroke cycle marine diesel engine during operating condition. The stresses were measured by means of wire and foil type electric resistance strain gauges. Electric connections between gauges and recorder were made through sliprings. Measured points were crank pin corner, web, connecting rod and crank pin bolt. Stress calculations by conventional methods were made on the basis of cylinder gas pressure and inertia forces of moving parts, taking into account of static test results. Good agreement was observed between calculated and measured stresses. Distance between support points of journal bearings seems to be constant under operating conditions, and no additional assumption was necessary. It is concluded that dynamic stresses of single-throw diesel engine crank shaft can be calculated with good accuracy by conventional methods already known.