The height of the velocity‐matching step (VM step) of a flux‐flow type Josephson oscillator is studied systematically by numerical simulations. It is defined by the phase velocity of the propagating fluxons equaling that of the electromagnetic wave in the junction structure. The height of VM step (&ggr;m) is an important parameter for the practical application of such a device. The results presented on &ggr;mare important data in order to save computer time in the simulation of electromagnetic properties, such as the output voltage, at the velocity‐matching condition. The detailed simulations were performed using the perturbed sine‐Gordon equation (PSGE) with a uniformly distributed bias current &ggr;. The PSGE was solved numerically by using an implicit finite‐difference scheme. The distributions of electromagnetic fields in an overlap, long Josephson junction in the flux‐flow state are mapped. The parameters influencing &ggr;mare discussed. We show that &ggr;mincreases almost linearly with an external magnetic field &bgr;eor with the quasiparticle damping &agr;, while it is less dependent on surface loss &bgr;. &ggr;mincreases slowly with junction length above a minimum length, and it levels out at long lengths. We also present an approximate expression for &ggr;mwhich fits numerical results well.