Since ancient times in China, decocting times have been adjusted to improving objective medicinal effects. In medical texts, decocting times are dictated by the amount of water that is reduced. And in this report, we surveyed rhubarb (Chinese, Da-huang ; Japanese, Daio) formulation decocting times described in the earliest Shang-han-lun and Jin-gui-yao-lue texts, from the amount of water changed. We found that rhubarb decocting times could be divided into four 10-, 20-, 30- and 50-minute groups when rhubarb is decocted with other crude drugs, while when rhubarb is put in water after other crude drugs, decocting times could be divided into four 1-, 5-, 10- and 20-minute groups. Next, when we prepared rhubarb decoctions using unprocessed rhubarb and processed rhubarb, and compared change in the principal compounds eluted into those decoctions, we found that 80% or more of the principal compounds were eluted into a decoction when either unprocessed or processed rhubarb were decocted for 30 minutes. It was therefore clear that, in ancient times, rhubarb formulations were decocted for 10 to 30 minutes so that not all of the principle compounds would elute, or given 50 minutes to allow all the principle compounds to elute. Meanwhile, it was also clear that rhubarb was put in water1to 20 minutes before decocting finished, to adjust the amount of principle compound eluted.