This article introduces the research results of a site index classification for Acacia hybrid plantations in Dong Nai Province. The objectives of this study were to (i) determine a baseline age of Acacia hybrid plantations to establish their site indices and (ii) develop site index curves for Acacia hybrid plantations. Three standard plots were established for each age group of 1-10 years with 111 trees per plot; 108 trees were measured for the estimation of growth criteria. Three trees were used for tree truncation, and truncated trees did not count for the estimation of tree growth criteria. In this study, the site index (SI) for Acacia hybrid plantations was divided into three levels according to the total height of the dominant trees. The heights collected from 108 trees were used to build the functions of the SI, and three truncated trees were used to examine the possibilities of the functions of SI. Research results showed that the appropriate baseline age of Acacia hybrid plantations at Dong Nai Province is 8 years. The site indices of hybrid plantations were divided into three site levels of I, II, and III, corresponding to heights of 24, 20 and 16 m, respectively. To improve the effectiveness of Acacia hybrid plantation businesses, owners should focus on growing plantations at site index levels of I or II.