Abstract: Based on Jones matrix,the property of twisted nematic liquid crystal(TNLC) is studied.It is found that TNLC modulated by voltage is equivalent to λ/2 wave-plate with proper thickness.Furthermore combination TNLC with two λ/4 wave-plate can be equivalent to achromatic λ/4 or λ/2 wave-plate wholly when the voltage is higher than liminal voltage,or it can be used as a common 90° rotationer when the voltage is removed.