A mathematical model to predict shape evolution during through-mask electrochemical micromachining (EMM) has been developed. Boundary element method has been used to solve the Laplace equation for electric potential with appropriate boundary conditions that describe the metal dissolution process under ohmic control. The influence of mask wall angle on shape of the evolving cavity, current distribution within the cavity and etch factor have been determined. For mask wall angles less than 90 degrees, the etch factor increased due to the shadowing effect of the mask, whereas the etch factor decreased for mask wall angles greater than 90 degrees. The influence of mask wall angle has been found to diminish with increasing metal film thickness.