OBJECTIVE To compare the efficacy and safety of oral chloral hydrate and sublingual midazolam to sedate the children undergoing echocardiography. METHODS A double-blind, randomized trial study in the children judged to require sedation prior echocardiogram were performed. Two hundred sixty-four patients between 6 months and 5 years of age were randomized to chloral hydrate or midazolam groups. Either 50 mg/kg of chloral hydrate orally or 0.3 mg/kg of midazolam sublingually was given in each groups. If the child was not responded within 30 minutes after the first dose, another half dose of each drug for the second dose will be required. The action duration time, sedation score level and the ability to complete echocardiogram were collected. RESULTS Both groups were comparable with respect to age, sex, body weight, underlying heart disease, baseline O2 saturation and functional heart classification. The children in chloral hydrate group needed the second dose for sedation more than midazolam group (10.6%, 5.3% p = 0.111). The onset, action duration and total study time were significantly shorter in midazolam than in chloral hydrate group (p < 0.001). The number of the patients who had the action duration within the optimal time (< 45 min) were significantly more cases in midazolam than in chloral hydrate group (93.1%, 43.5% p < 0.001). Success rate of echocardiogram was 99.2% in each group. There was no difference in echocardiographic time performed in both groups. The children in chloral hydrate group had deeper in level of sedation (p < 0.001). Both groups showed no significant difference in term of the ability to complete echocardiographic examination. The reaction of the children to take the medication and the number of the patients who had systemic O2 saturation change more than 5%from the baseline were higher in chloral hydrate group significantly (14.4%, 4.5% p = 0.006 and 9.9%, 3.1% p = 0.025). CONCLUSIONS Sublingual midazolam at the dose of 0.3 mg/kg can be used to sedate the children at age group between 6 months to 5 years who undergoing echocardiogram with comparable rate of success and safety as 50 mg/kg of chloral hydrate orally. The less depth in the level of consciousness after sedation with midazolam compare to chloral hydrate may be advantage in a high risk patient to avoid deep sedation but may be disadvantage in case who need more comprehensive echocardiographic evaluation.