Natural indigo dye in powder form was prepared by modifying traditional Niram method, using instead of baked oyster powder for precipitating indigo dye. The prepared dye was applied to dyeing Tencel fabrics to investigate the effect of experimental conditions for the optimization of dyeing process. The indigo dye powder contained 15.2%(w/w) of indigo content and 0.757%(w/w) of indirubin content on the basis of HPLC analysis. Maximum dye uptake was obtained at for 20min. Almost saturated dye uptake was obtained at 2g/L of sodium hydrosulfite concentration up to 4g/L of indigo dye and then slowly increased for further increase of sodium hydrosulfite. Whereas at higher indigo dye concentration(8g/L) more than 3g/L of reducing agent concentration was required for obtaining the maximum dye uptake. At the same indigo dye and reducing agent concentration, K/S value of the sample dyed without sodium hydroxide(pH 5.75) was 15.19, much higher than one dyed in alkaline condition(K/S 5.76). There was no difference in colorfastness ratings among samples with different color strength. However, more fading was occurred for the sample with low color strength.