This study was carried out to find out the optimum method of preparation of indigo standard solution and its stability, and to investigate the indigo contents in Niram, blue dye extract, from a total of 7 indigo plants and 34 breeding lines of Persicaria tinctoria H. Gross. Proper solvent for indigo standard was dimethyl sulfoxide (DMSO), and appropriate concentration was 1 mg of indigo in 10 mL of DMSO. Absorbance value of UV/Vis Spectrophotometer at 620 nm of standard solution was changed decreasingly 12 hours after the preparation of standard solution irrespective of the storage conditions such as temperature and light. Average value of absorbance of 8-fold diluted standard solutions prepared daily during 16 days was $0.210{pm}0.005$, indicating the powder of indigo compound was stable chemically. Calibration curve was made for quantitative analysis of indigo of 7 Niram samples, and indigo contents ranged from 0.69% to 18.76% showing relatively larger variation. Across all 34 breeding lines, the range of indigo content was from 7.9 mg to 56.4 mg per 100 g of fresh leaves, averaging 25.2 mg of indigo content and showing a 47.7% coefficient of variation.