Employing explicit expressions of Green's functions for an anisotropic elastic half-space, the image force exerted on a line dislocation in the half-space is obtained. When the boundary of the half-space is traction free, the image force obtained agrees with that existing in the literature. We also obtain the image force for the case in which the boundary is fixed (i.e. clamped). In the special case of isotropic materials the result reduces to that available in the literature. While the image force for the traction-free boundary always tends to attract the dislocation to the boundary, the image force for the fixed boundary is repulsive. Moreover, the magnitude of this repulsive force is always equal to or larger than that of the attractive force associated with a free boundary. We also show how the image force for the half-space can be deduced from the image force for two joined dissimilar half-spaces. The image force is independent of the orientation of the half-space boundary once the Burgers vector of the dislocation is prescribed, provided that the boundary is parallel to the x3-axis and the distance between the dislocation and the boundary is not altered. This agrees with the more general case of the image force theorem for two joined dissimilar half-spaces, one of which is dislocated.