In previous paper a new technique for measuring protrusion height of abrasive grain was proposed to estimate the grinding performance of diamond wheel. This paper describes the truing mechanism of bond matrix type wheel focusing on lapping effect, removal of bonding material around cutting edge by isolated grains torn off from GC cup-truer, in a metal bonded diamond wheel truing. The main results obtained are as follows : (1) Lapping effect plays an important role in truing process, therefore the truing as well as dressing can be performed in the same apparatus at the same time by using the GC cup-truer. (2) The truing ratio can reach its peak value through the control of the flow of grinding fluid against outflow of isolated grains. (3) The mean protrusion height depends only on the truing conditions after an enough truing and dressing operation. (4) An increase in size of isolated GC grains promotes the lapping effect in truing process and results in the growth of the truing ratio and the mean protrusion height.