Exchange coupling and magnetization reversal mechanism in two series of Cox Ni1-x/CoO (30 nm) (x=0.2 and 0.4) bilayers are studied by vector magnetometer. Two components of magnetization are measured parallel and perpendicular to the applied field. At low temperatures, coercivity He ∝ (tFM)-n, n = 1.5 and 1.38 for x = 0.2 and 0.4, respectively, in agreement with the random field model. At room temperature, the coercivity is nearly proportional to the inverse FM layer thickness. In addition to the exchange field and the coercivity, the characteristic of the magnetization reversal mechanism was found to change with temperature. At temperatures below 180 K, magnetization reversal process along the unidirectional axis is accompanied only by nucleation and pinning of domain wall while magnetization rotation is also involved at high temperatures.