In the measurement of patterned resists, deviation of the critical dimension (CD) is frequently observed. We have found that contamination on the resist on a Si wafer can degrade the static repeatability more than the charging phenomenon at low accelerating voltage (Vacc), that is, near 1 kV. We investigate the dependence of contamination rate (Rc) and the normalized yield of secondary electrons (δ’) on Vacc and emission current (Ie) in order to clarify the influence of the contamination and charging effects. Experimental results indicate that we can minimize the Rc by optimizing Vacc and Ie. The Rc change cannot be attributed only to the contamination and charging effects, however. It seems, instead, that two competitive reactions (deposition and etching) occur simultaneously during the measurement at low voltage. The etching effect can be interpreted as a dissociation reaction at the resist surface. To reduce the CD deviation and improve the accuracy of the CD measurement, the accelerating voltage and current density must be optimized. Doing so improves the static repeatability from 0.008 to 0.003 μm (3σ).