Abstract: Background and Objective: Eastern Hoolock Gibbon Hoolock leuconedys is a threatened species categorized as "Vulnerable" in the IUCN Red List of threatened species. It has narrow distribution range in the foothill forest of Eastern India, China and Myanmar. Besides, few works on its population distribution, feeding behaviour and conservation issues, there has been no research regarding its reproductive behaviour conducted so far. The aim of the study was to explore the various reproductive aspects and other behaviours of H. leuconedys. Materials and Methods: Six focal groups of H. leuconedys residing in free-ranging enclosures were selected to observe their sexual and reproductive behaviours from July, 2011-December, 2016. Ad libitum and focal animal techniques were used to record its various reproductive parameters such as sexual solicitation, mating postures, gestation length after gestation length birth interval etc. Statistically data was analyzed by percentage, mean and standard error. Results: During the study period, 94 mating attempts were recorded. Based on number of attempts/day H. leuconedys was distinguished as "Single mount ejaculator." Mating was throughout the year with maximum attempts in January and minimum in July and October. Mating events occurred between 0800-1200 h. Of the 91 mating events, 39 events of solicitation were recorded in which female or male or both solicited each other before mating. Record of a rare posture "Female superior position," along with dorsoventral and ventroventral postures was also noted. The average duration of each copulation was 28.4±1.2 sec. The gestation length was 189±0.92 days. Two birth seasonality were observed, first from June to December and second in March. Two distinct inter birth intervals were estimated based on infant survival (3.1±0.3 years) and infant mortality (1.5±0.2 years) just after infant birth. The average birth rate of five groups was 0.42±0.06 birth/female/year. Conclusion: The present study provides information on the various reproductive aspects of Hoolock leuconedys such as mating behaviour, mating period, birth seasonality, gestation length, birth interval, post-copulatory behaviours, etc., that would be useful for conservation of this threatened species. This information will also be useful for preparation of husbandry manual for gibbon to act as a guiding tool for gibbon management in ex situ condition.