Abstract-By using some special passive structures and correction of boundary conditions a novel method to improve the electro-magnetic simulation accuracy is proposed. With this method many substrate parameters, such as thickness, loss, dielectric constant, loss tangent, sheet resistance, square capacitance and conductivity of the metal, can be described more accurately, and many high frequency effects caused by skin effects, parasitic effects, coupling between microstrip lines and fluctuation from the sheet resistance, etc. can also be simulated more precisely. An accurately scalable small-signal model for millimeter wave HEMTs is implemented and presented. Combined with distributed modeling, pulsed IV and S parameter measurements, this model can be made scalable freely. The measurements agree with simulated results very well, which also shows that this method applied to the scalable small signal model has a good consistency and accuracy.