Kosumpee Forest Park (KFP) is located in Northeast Thailand and is home to a resident population of long-tailed macaques (Macaca fascicularis). This study analyzed the genetic diversity of the KFP population based on random amplified polymorphic DNA technique by the Shannon-Weinner index of diversity (H') using the NTSYS program. Blood samples were collected from two of the social groups (RedDot group and HareLip group), and 16 successful primers produced 143 loci with 74.12% polymorphism. The H' value of the population was 3.30 and the genetic evenness was 0.97. A UPGMA dendrogram divided the samples into three distinct clusters base on pelage color: the first cluster consisted of individuals with gray pelage from both social groups, the second included individuals with yellow-gray and yellow pelage from both social groups, and the third included individuals with gray pelage from only the HareLip group. The H' values for the gray, yellow-gray, and yellow pelage groupings were 2.83, 2.30 and 1.10, respectively. The genetic similarity values ranged from 55 to 96%. Genetic diversity analysis indicated that yellow-gray macaques were genetically similar to yellow macaques. The results of this study indicate that the macaque population at KFP currently possesses substantial genetic diversity. To our knowledge, this paper represents the first report of genetic diversity in this population, and as such, provides an important baseline measure for future comparison.