Background: Aquilaria crassna Pierre ex Lecomte (Thymelaeaceae) has been used as a medicinal plant in many aspects. Previous research has revealed that A. crassna leaves contain mangiferin as an active compound. Although the active component has been investigated, the pharmacognostic specification and quantification of mangiferin from A. crassna leaves have never been established. Objective: The current study aimed to conduct and develop a pharmacognostic standard according to WHO guidance as well as the validated method for quantifying mangiferin content. Materials and Methods: Dried A. crassna leaves from 15 separated locations throughout Thailand were investigated for pharmacognostic specification. Their mangiferin contents were quantitatively analysed by TLC densitometry with win CATS software. Results: Macroscopic-, microscopic- characteristics and TLC fingerprinting combined with physicochemical parameters were reported in this study. The loss on drying, moisture content, and total ash content as well as acid-insoluble ash content were determined to be 8.62 ± 0.13, 8.16 ± 0.14, 6.82 ± 0.09 and 1.49 ± 0.03%, respectively. Ethanol- and waterextractive values were found to be 9.05 ± 0.39 and 16.94 ± 0.22 %, respectively. In addition, the validation method for quantifying the mangiferin content was developed. The contents of mangiferin in A. crassna leaf extract determined by TLC-densitometry and TLC-image analysis were found to be 1.2992 ± 0.5980 and 1.3036 ± 0.5874 % by dried weight, respectively. The results between these two analytical methods were shown to have an insignificant difference. Conclusion: This study provides the necessary information for authentication and standardisation of A. crassna leaves.