Through experiments and theoretical analysis, we investigated the similarity of gas discharge in low-pressure Argon gaps between two plane-parallel electrodes. We found that the breakdown voltages depended not only on gap length and the product of gas pressureand gap length but also on the aspect ratio of the gap, i.e. Ub = f (pd, d/r). When we considered similar discharge gaps, the radius r, gap length d, and gas pressure p fulfilled the conditions of p1 r1 = p2 r2 and p1d1 = p2 d2. In this situation, the reduced field E/p was also constant. The voltage-current characteristic curves of similar gaps were approximately the same, which is a novel experimental result. Comparison of the discharge physical parameters of the scaled-down gap and prototype gap shows that the proportional relations can be derived from the similarity law. Our experimental results provide some instructions on extrapolating two similar gaps and their discharge properties. Application of the similarity law is straightforward when we scale the discharges up or down if they are too small or large.