Abstract: The advancing front theory is an approximate solution for mass transfer of O2 within blood when the reaction of O2 with homoglobin can be assumed to be very fast. An approximate solution for the Graetz and Leveque problems from the advancing front theory is modified according to the analogy between momentum and mass transfer in this study. By substituting third order polynomial for linear relationship of O2 concentration profile in the saturated region, the deviation of the modified approximate solution from that of exact solution is decreased to 4.36% from Stroeves 15.1%.