This paper investigates the yaw dynamic behaviour of a seven axle tractor semitrailer combination vehicle developed by VRDE (Vehicle Research & Development). The semitrailer has four steerable axles which follow command steering law i.e. all axles of semitrailer are steered in a particular relation with articulation of tractor. A 4 dof (degree of freedom) linear yaw plane model was developed for this combination vehicle. Yaw response characteristics such as lateral acceleration, yaw rate and articulation angle for step and sine steer is obtained from this model. Effects of speed on the above parameters are also studied to the same steering inputs. Lateral tyre forces due to semitrailer steering at various speeds are estimated to understand its distribution on each axle. Steady state yaw rate and articulation angle gain are obtained to predict the understeer / oversteer behaviour of combination vehicle. Eigenvalue analysis carried out to identify the modes responsible for lateral stability of combination vehicle. Limit speed of the combination vehicle is obtained from damping ratio verses forward speed analysis. The actual prototype semitrailer with steerable axles has been developed, realized and experiment conducted for single lane change manoeuvre of this combination vehicle. Yaw rate response was measured at CG (centre of gravity) location of tractor and semitrailer using yaw sensors during the manoeuvre. The developed mathematical model has been correlated and validated with experimental results.