A capacitor is one of the most widely used electronic elements.Difference between the nominal value and the actual value of a capacitor occurs due to edge effect,which may cause unexpected breakdowns of the capacitors.Two kinds of rectangular parallel-plate capacitors with non-ideal edges are analyzed in the paper.The semi-quantitative capacitances in capacitors with non-ideal edges are determined.The paper calculates the quantitatihe capacitances in plate capacitors uhich are not strictly paralleled.Some noticeable relevant issues in practical applications are proposed.