The paper present extensions of some results developed in the parametric robust control to fractional order interval control systems (FOICS). Computation of the Bode and Nyquist envelopes of FOICS are studied. Using the geometric structure of the value set of fractional order interval polynomials (FOIP), a technique is proposed for computing the Bode and Nyquist envelopes of transfer functions whose numerator and denominator polynomials are fractional order polynomials with interval uncertainty structure. The results obtained are useful for the analysis and design of FOICS. Numerical examples are included to illustrate the benefit of the method presented.