This study examined factors associated with diarrhea and ARI among children aged less than 5 years in Thailand using data from the MICS conducted from December 2005 to February 2006. A stratified two stage sampling technique was used with provinces constituting strata. Primary sampling units were blocks for municipal areas or villages for non municipal areas. Sample selection of blocks/villages was done using probability proportional to size. Secondary sampling units were systematically sampled individual households in each block/village. Data were collected for 9444 children with 9409 children who had complete data on age being included in the analysis. Bivariate analysis was done using Chi square and Fisher’s exact tests with multivariate analysis being done using binary logistic regression. The study found increased risk of diarrhea among children in households with one child under 5 years or with Thai heads. Male children and those with elderly caretakers were also at increased risk of diarrhea. The highest risk of diarrhea was in children aged 6-23 months. Household wealth index quintile also had an association with diarrhea with children in the poor, middle and fourth wealth index quintiles being at increased risk of diarrhea compared to children in the richest wealth index quintile. Unsafe disposal of child’s feces and not breastfeeding at all were associated with increased risk of diarrhea in children aged less than 2 years. As for ARI, living in rural areas, living in North East region, having a Thai household head, being a male child, being aged 13-23 months and having never breastfed were associated with increased risk of ARI in children aged less than 5 years. Not being breastfed was associated with increased risk of ARI in children aged less than 2 years. Continued efforts to promote recommended child feeding practices, providing health education to targeted populations on hygienic practices in child care, special attention in the care of male children and those aged 6-23 months, targeting teenage and elderly child caretakers in diarrhea prevention efforts and longitudinal studies on diarrhea and ARI in children are recommended.