We have examined applying low-frequency electric field to the liquid crystal cell at the phase transition temperature from nematic to smectic. The cell treated by applying 10 Hz,±5 V voltage for 3 minutes reduced zig-zag defects and obtained a well-aligned mono-domain smectic C* phase. It was difficult to obtain a mono-domain for ferroelectric liquid crystal without smectic A phase. We obtained high contrast ratio of 120:1, continuous gray scales and high-speed response of 7 ms in ferroelectric liquid crystal with smectic A phase. And the treated twisted smectic C* cell exhibits mono-stable behaviour. We found that it is essential to drive the cell practically by AC voltage. Because the residual polarization is removed and the molecule is returned rapidly to the twisted state by applying offset voltage. The response speed of mono-stable operation was 770μs.