The paper presents a fuzzy model of the evaluation activity within a large organisation. The model was developed for the evaluation of competencies of professional workers. The model was developed in two phases. The aim of the first phase was to define the set of competencies and their explicative force in the evaluation process. According to the model, the evaluation is carried on by a network of evaluators, who evaluate the competencies expressed by individuals in situations they manage during their activity. The evaluator are forced to explain the motivation of their judgement. The resulting evaluation discourse is represent as a tree, where the root is the final judgement and the leafs are the final categories justifying the judgement. The second phase aimed to use the trees in order to design an evaluation procedure. At this end the tree are interpreted by using the fuzzy logic. The model let an user to explore the evaluation problem from different points of view, making his decisions more reliable. The model was tested in a large corporation. The logic structure of the model could be easily generalised to other evaluation issues, such as quality management, maintenance, operation management, subcontracting, human resource management.