For a class of control systems with disc uncertainties, robust performance analysis is developed. On the strictly positive realness and H-infinity-norm of uncertain systems, from the geometric point of view, two new sufficient and necessary conditions are given. The largest H-infinity-norm bound, containing the coefficients of only stable polynomials and centered at a nominal stable point in the coecient space is found. The results obtained in the paper are tractable and concise, which is illustrated by some numerical examples.