Delay estimation is a difficult problem in computer networks. RTT (Round Trip Time) is often used as an approximation of the delay, but because it is a sum of the forward and reverse delays, the actual one-way delay cannot be estimated accurately from RTT. Accurate one-way delay estimation becomes crucial because it serves a very important role in network and application design. This paper proposes a new scheme to estimate one-way delay and its variation. The scheme calibrates estimated one-way delay so in a brief duration as to be used in many protocols that adjust their behavior depending on the network condition. We analytically derive one-way delay, forward and reverse delay respectively, and show that our one-way delay estimation is much more accurate than RTT estimation by simulation.