In this paper I investigate factors of the testability of structured programs. The starting point is given by a study of the literature to obtain both an initial model of testability and existing models related to testability. The main objective of this paper is to propose models for the estimation of testability (Tb), on the basis of various affecting attributes. The values of the attributes were estimated for programs under consideration with the help of software developed for this purpose. The models proposed conform to our intuitive reasoning.