<p>This paper is mainly devoted to the crossing period annulus and the maximum number of critical periods for planar piecewise linear systems with a straight line of separation. The normal forms of such systems with a crossing period annulus are obtained by an admissible transformation with a linear time scaling, which shows that the systems after transformation are reversible with respect to one of the axes. By further analyzing the monotonicity of the period functions of these normal forms, we prove that the piecewise linear systems with a straight separation line have at most one critical period, which is also sharp.</p>