Inhibitory concentration (IC50) of 27 isolates of Plasmodium falciparum collected from Mae Sot, Tak Province to Isoquine, Amodiaquine, Desethylamodiaquine and Chloroquine were found to be ranged from 0.8-9.9 nM, 1.8-14.1 nM, 1.9-11.8 nM, and 6.0-118.4 nM respectively. A significant positive correlation was found between the IC50 values of chloroquine and amodiaquine (r = 0.84, P