In this study, we have investigated properties and behaviors of the indigo vat dyeings on synthetic fibers, namely PET and PTT/Spandex. In addition, indigo vat dyeing conditions such as dyeing temperature, dye concentration and pH were optimized. The finding results show that higher color strengths of indigo dyeings on the two types of applied fibers were obtained at $110^{circ}C$ and $90^{circ}C$, respectively. Furthermore, acid leuco dyeings on the fiber substrates using acetic acid and formic acid show higher dye uptake with compared to alkali leuco counterparts.