Substandard and counterfeit pharmaceutical products, including antimalarial drugs, appear to be widespread internationally and affect both the developing and developed countries. The aim of the study was to investigate the quality of antimalarial drugs, ie, artesunate (ART), chloroquine (CHL), mefloquine (MEF), quinine (QUI), sulfadoxine/pyrimethamine (S/P) and tetracycline (TT) obtained from the government sector and private pharmacies in 4 Thai provinces: Mae Hong Son, Kanchanaburi, Ranong, and Chanthaburi. Three hundred sixty-nine samples of 6 antimalarial drugs from 27 government hospitals, 27 malaria clinics, and 53 drugstores, were collected. Drug quality was assessed by simple disintegration test and semi-quantitative thinlayer chromatography in each province; 10% passed, 100% failed and doubtful samples were sent to be verified by high performance liquid chromatography (HPLC) at the Thai National Drug Analysis Laboratory,(NL). Fifteen point four percent of ART, 11.1% of CHL and 29.4% of QUI were substandard. Based on the finding, drug regulatory authorities in the country took appropriate action against violators to ensure that antimalarial drugs consumed by malaria patients are of good quality. national capacity in monitoring antimalarial drug quality as part of the antimalarial drug assurance (qA) system, and to investigate the quality of artesunate (ART), chloroquine (CHL), mefloquine (MEF), quinine (QUI), sulfadoxine/pyrimethamine (S/P), and tetracycline (TT).