The size-dependent transverse electric field ratio (i.e. transverse field to longitudinal field, or simply tan alpha ) of zinc was measured by an improved virtual probe method at 77K. The reliability of the measurement was checked by measuring the electrical anisotropy of resistivity of a 60 degrees orientated sample and by measuring the tan alpha of a 0 degrees and 90 degrees oriented sample. The inverse proportion of tan alpha with respect to sample thickness found at 77K can be explained in terms of a diffusion size effect model.