  Supersymmetry is formulated for integrable models based on the $sl(2|1)$ loop
algebra endowed with a principal gradation. The symmetry transformations which
have half-integer grades generate supersymmetry. The $sl(2|1)$ loop algebra
leads to N=2 supersymmetric mKdV and sinh-Gordon equations. The corresponding
N=1 mKdV and sinh-Gordon equations are obtained via reduction induced by
twisted automorphism. Our method allows for a description of a non-local
symmetry structure of supersymmetric integrable models.
