The interpolation mechanism of the single-input-single-output (SISO) fuzzy controller is further analyzed.Based on the symbol calculation,analytic expressions for the control function of the SISO fuzzy controller and the differences between the control function and the interpolation expression are derived,when the implication operator is specified as the min operator or the multiply operator.Then an upper bound estimation of the differences is given and the number of the fuzzy inference rules is selected.