We use an advection hypothesis to analyze the decay of centerline velocity of a circular jet issuing into a counterflowing stream. Working in the Lagrangian frame, we follow the locations and velocity gradients of jet fluid particles along the jet central axis while the particles are being advected backwards by the counterflow. The spatial velocity gradient along the jet centerline is thus obtained and subsequently integrated to describe the spatial decay of axial velocities. Laser-doppler velocity measurements are performed in the laboratory and the data are well predicted by our analytical expression of centerline velocity decay. Looking from another view, our treatment supports that the effect of an external axial flow stream on the jet flow field can be represented by a certain degree of stretching or contracting of the jet in the axial direction. ©1998 American Institute of Physics.