Temperature dependence of thermoelectric power S in quasi-two dimensional potassium/sodium purple bronzes has been studied in the temperature range 80—280K.It′s found that:(1)in potassium purple bronze,S has a sign change at 105K,above which S is the small negative value and can be described by the form of S=AT+B,the dominant carriers are free electrons,in the CDW state below 105K,S is positive,and can be described by S=A′T+B′/T,the phonon-drag of the carriers is dominant; (2)in sodium purple bronze,in the studied temperature range above 80K,S has a good linear relation with temperature,no phase transition was observed,electrons may be the dominant carriers.In addition,the origins of the difference of the properties for both these isostructural crystals are also discussed briefly.