In this paper, we develop the theory of so-called nonstandard Gaussian quadrature formulae based on operator values for a general family of linear operators, acting of the space of algebraic polynomials, such that the degrees of polynomials are preserved. Also, we propose a stable numerical algorithm for constructing such quadrature formulae. In particular, for some special classes of linear operators we obtain interesting explicit results connected with theory of orthogonal polynomials.Keywords Gaussian quadrature · Interval quadrature · Linear operator · Zeros · Weight · Measure · Degree of exactness · Orthogonal polynomial · Linear functional Mathematics Subject Classifications (2000) 41A55 · 33C45 · 33D45 · 65D30 · 65D32