Nine plant species namely greater galangale (Alpinia galanga ), fingerroot (Boesenbergia pandurata ), turmeric (Curcuma longa ), cardamom (Elettaria cadamomum ), neem (Azadirachta indica ), Siamese cassia (Cassia siamea ), citronella grass (Cymbopogon nardus ), eucalyptus (Eucalyptus citriodora ) and Siam weed (Eupatorium odoratum ) were studied for potential efficacy in reducing human-mosquito contact when used in mosquito coils. Each plant was prepared as 25% ingredient in each mosquito coil formulation. Efficacy of the nine mosquito coil formulations and blank coil against night biting mosquito were determined under field conditions in an urban area of Nonthaburi Province, Thailand. Evaluations were carried out by capturing landing-biting mosquitoes on human volunteers and were conducted from 1830 h to 2130 h. Results showed that mosquito coils provided protection against mosquitoes with a range from 50% to 71% reduction in biting activity while the blank coil consisting of inert materials only reduced mosquito attacks reduction in biting activity while the blank coil consisting of inert materials only reduced mosquito attacks by about 43%.Mosquito coil containing leaves of citronella grass showed highest efficacy whereas that containing rhizomes of turmeric was least effective. Mosquitoes caught in this study included 12 species belonging to 5 genera (Aedes, Anopheles, Armigeres, Culex and Mansonia), but Culex gelidus , Cx. quinquefasciatus, Cx. vishnui  and Mansonia indiana  were found to be the most predominant species.