The study of Knowledge and Understanding on Personal Income Tax Return of the XXX Company Limited, Chon Buri Province was conducted. The study purpose was to study the knowledge and understanding about personal income tax return of the XXX Company Limited. Questionnaires were used as the data gathering instrument in this study. The sample used in the study comprised 96 staff of the XXX Company Limited. Descriptive statistics was used for the data analysis which comprised of percentage, mean and standard deviation.