Abstract: TRL calibration, one of the most commonly used calibration method, is to calibrate the VNA by measuring three standards through (T), reflect (R) and line (L). In practical applications, the calibration standards are not ideal in performance. Such non-ideal calibration elements will lead to deviations of error terms and, consequently, the uncertainty of measured S-parameter. As an example of multi-port VNA, the deviations of the measured S-parameters of 4-port VNA are presented as functions of the deviations of the calibration elements parameters from their ideal values. Using the concept of general node equation, a matrix formula for the S-parameter deviations with respect to the error terms has been deduced. In addition, expressions representing the deviations of error terms with respect to the non-ideal calibration standards are given by series of matrix operations. Finally, after calculation of sensitivity coefficients, they can be used for establishing the type-B uncertainty budget for S-parameter measurements.