This brief studies the current mismatch of MOS transistors operating in weak inversion region. Explicit formulas are derived from the drain-current relationship of a long-channel MOS transistor. By referring the drain-current variance to the gate terminal under the small-signal conditions, the dependence of the gate-source voltage on the design parameters and sources of mismatch can be considered. This will help designers to choose the optimum sizes and the efficient gm/ID ratio for current offset reduction of transistors that their mismatch affects the circuit performance, significantly. In order to confirm the accuracy of the calculated drain-current variance, the hand analysis results are compared with MATLAB simulator outputs. In addition, the input offset of a conventional folded-cascode amplifier is estimated by hand analysis, and the results are compared with SPICE simulator outputs through Monte Carlo analysis.