An anti-dust structure for a sliding block of a linear guideway comprises a sliding block, two end caps, plural rolling elements, a flexible anti-dust strip and a rail. The two end caps are assembled to both ends of the sliding block, the rolling elements are disposed on the rail. Between the end cap and the sliding block is defined a circulating path. The two end caps are each extended from an inner side thereof to form a linking portion, and the two linking portions of the two end caps are linked to each other to define an endless positioning groove including two longitudinal grooves and two transverse grooves. The flexible anti-dust strip is disposed in the positioning groove of the end caps and integrally formed with protruding lip portions for cooperating with the two longitudinal grooves.