The etch rate of 42 Alloy in iron (III) chloride etchant is measured in order to investigate the effects of agitation speed, etchant concentration, temperature, co-existing foreign metal ions and resist hole diameter. The rate increased at first and gradually decreased in accordance with increasing etchant concentration. The activation energies are 12 and 24-30 kJ/mol at a low concentration and at a higher concentration, respectively. Foreign metal ions in etchants significantly suppressed the etch rate. The etch rate increased with decreasing diameter of the resist hole due to the effect of side etch.