The effect of higher order strain gradients on adiabatic shear banding is investigated by considering the simple shearing of a heat-conducting thermoviscoplastic material with a gradient-dependent flow stress. The competition between the gradient-dependent plastic flow and heat conduction, and their influence on the shear band width and structure are examined. Two internal length scales, i.e. the deformation internal length and the thermal internal length, are incorporated into the linear stability analysis, which shows that the band width size scales either with the square root of the strain gradient coefficient (in the absence of heat conduction) or the thermal conductivity (in the absence of strain gradients), respectively. The numerical computation for the nonlinear problem reveals that the ``diffusive'' effect of the strain gradient is much stronger than that of the heat conduction and dictates the constitutive response of the material in the postlocalization regime, and shows that the deformation length scale is much larger than the thermal length scale. The band width predicted by the gradient theory agrees reasonably well with experimental observations found in the literature.