This paper deals with the measuring methods of flank wear, receding cutting edge of helical square end mill, the tool life equation and also the relationship between flank wear and accuracy, by end milling. The results are as follows. (1) The applicable measuring method of flank wear and decreasing diameter of helical end mill is given and the measurement is performed. (2) There are two types of flank wear, one is the rectangular type and the other is the rectangular type with the grooved wear. The grooved wear appears in down milling, and becomes bigger as the cutting speed increases. (3) The tool life equations are given with three kinds of tool material in up or down milling for a critical flank wear of 150 μm in the normal direction to the cutter axis. (4) Work accuracy depends upon receding a cutting edge. And the configulation of wear is projected on a side surface of a work. But such a projection rule cannot be maintained due to a grooved of the helical flute.