To clarify the anisotropy of the dc size effect in Al, measurements have been made of the electrical resistivity of high-purity Al single crystals at 4.2 K. The specimen surfaces were set parallel to each of three crystallographic planes, {100}, {111} and {110}, and the axis orientations were parallel to , and . The main results were the following. (1) The size effect increased in the following order of the surface orientations: {110}, {111}, {100}. (2) For the size effect due to a {110} surface, the Fuchs-Sondheimer theory with gave a good description, while, for the size effect due to {100} and {111} surfaces, substantial disagreement with the theory was suggested for the very thick specimens. (3) For each surface orientation, for sufficiently thin specimens was independent of the axis orientation within the experimental error; i.e. the size effect was independent of the direction of current flow. (4) However, as the specimen thickness increased, strong anisotropy of with respect to the current direction appeared: increased in the following order of directions: . Results (3) and (4) suggest an anisotropy effect of the bulk resistivity in high-purity Al.