Let P(s,δ) be a sphere plant family described by the transfer function set where the coefficients of the denominator and numerator polynomials are affine in a real uncertain parameter vector δ satisfying the Euclidean norm constraint ||δ||＜δ.The concept of stabilizability radius of P(s,δ) is introduced which is the norm bound δs for δ such that every member plant of P(s,δ) is stabilizable if and only if ||δ||＜δs.The stabilizability radius can be simply interpreted as the ‘largest sphere’ aroun d the nominal plant P(s,0) such that P(s,δ) is stabilizable.The numerical method and the analytical method are presented to solve the stabilizability radius calculation problem of the sphere plants.