In this paper, the effect of mis-alignment on path difference in stable plano-concave resonators (sjmply called plano-concave cavities afterwards), whioh are widely used in practice, has been discussed by means of matrix methods. The method is suitable to the common spherical cavities in principle. For convenience of analysis, however, the plano-concave resonators are only discussed within miniature mis-alignment. After comparison with experimental data and theoretical results three conclusions are obtained: (1) When the plano-concave resonators are correctly aligned, the effect of mis-alignment on path difference will be the most sensitive. (2) The plano-concave resonators have the "self-alignment" effect within miniature mis-alignment, and the experiment shows that the path difference will fluctuate with increase of mis-alignraent angle. (3) If the angles of mis-alignment are less than 1", the effect on path difference for plano-concave resonators drop down to 3 χ 10一9, and if the angles of mis-alignment could be further reduced to 0.2", the effect on path difference will drop down to 1×10 10.