Several attempts have been reported in the past decade or so which measured the sizes of particles in lubricant oil in order to monitor sliding conditions (1). Laser light extinction is typically used for the measurement. It would be an ideal if only wear debris particles in lubricant oil could be measured. However, in addition to wear debris, particles such as air bubbles, sludge and foreign contaminants in lubricant oil are also measured. The wear debris particles couldn't have been separated from other particles, and therefore this method couldn't have been applied to measurement devices for detection when maintenance service is required and how the wear state goes on.It is not possible to grasp the abnormal wear in real time by the conventional techniques such as intermittent Ferro graphic analysis. In addition, it is no way to detect which particle size to be measured by the particle counter alone. In this study, we attempted to detect abnormal engine operations by in-situ monitoring of particles in lubricant. In order to differentiate wear particles from above-mentioned other ones, we took the following original approach. Wear debris particles in lubricant oil during stable wear state and abnormal wear state are observed by scanning electron microscope (SEM) and ferrography and the particular sizes of wear debris particles are expected. The sizes of particles which might be released in an abnormal engine operation are measured by in-situ particle counting. In this experiment, one of the four cylinders of an engine flamed out and then wear debris particles of the predicted size were observed. As a result, it was found out that there was a correlation between the particle size distribution and the engine condition.