α-mangostin is a xanthon class that has been reported having antibacterial activity, antiproliferative, antiinflammatory and antioxidant. α-mangostin is found in many plant of Clusiaceae family, one of them is Garcinia mangostana L. The aim of this study is to find analysis method of α-mangostin from dichloromethane extract of green fruit latex G. mangostana L. that is validated and to determine quantity of α-mangostin from this extract by High Performance Liquid Chromatography. Column that used was Shim-pack® VP- ODS 250 x 4.6 mm, UV-Vis detector SPD M-20A Diode Array with wavelength 244 nm, mobile phase methanol : formic acid 0,4 % (86: 14), a flow rate of 1 mL/min, isocratic system and injection volume 20 uL. The result showed LOD value is 0.345 mg/mL and the LOQ is 1.151 mg/mL with r = 0.999, intraday precision with RSD values 0.732%; 0.188%; 1.827%, interday precision with RSD value 1.228%; 1.359%; 1.472%, and the recovery of the value is 100.832%; 98.008% and 99.680%. The content of α-mangostin from dichloromethane extract of green fruit latex G.mangostana L. is 49.869 %.