The purpose of this study was to find out the relationship between mothers knowledge and self care practices to their children on ARI and the severity of acute respiratory infections among children under five years of age. A hospital based cross sectional study was conducted in two hospital (maternal and children hospital and Ratchaburi regional hospital) at Ratchaburi in Thailand. Total one hundred and eighty mothers of under five children with acute respiratory infections who came to the hospitals during the period from 1st April to 23 April were interviewed using by structured questionnaires. After analyzing data using epi-info and spss/pc program of computer, it was found that 20% of mothers had good knowledge, 39.4% had fair knowledge and 40.6% of mothers had poor knowledge on ARI. It was also seen that the majority of mothers (58.3%) had fair practice to their children, 18.3% of mothers had good practice and 23.3% of mothers had poor practice to their children. The occupation of mothers of ill children were mostly house wife (50%) and educational level were mostly either primary education or no education (66.1%) The study results showed that mothers knowledge and self care practice were significantly associated with the severity of acute respiratory infections. (P = .03 and .003). It was also found that mothers knowledge significantly associated with the mothers self care practice on ARI. (P = .00006) The study revealed that mothers age and educational status were significantly associated with the severity of ARI (P =.02 and .01). In this study a significant association was found between the mothers self care practice and the delayed medical care seeking. (P = .03). The study revealed that children immunization status and breast feeding status were significantly associated with the severity of ARI (P =.04 and .036). A significant association also found between the delayed medical care seeking and the severity of ARI. (P = .00026). The study results did not show any statistically significant association between the environmental factors and the severity of ARI. The study revealed that sociodemographic factors was not significantly associated with the mothers knowledge and practice on ARI. An additional finding in this study was that in 66% of the families where at least one smoking members were living with the under five children which was a major risk factors for the occurrence of ARI. It was recommended that health education should be enhance to raise the mothers knowledges for good care of their children. Besides these the activities of the health personnels should be more integrated with this aspect.