A model was formulated for developing functional illustrations for text-based competency-based vocational education (CBVE) instructional materials. The proposed model contained four prescriptive steps that address the events of instruction to be provided or supported and the locations, content, and learning cues for each illustration. Usefulness of the illustration design model was evaluated by using the model to illustrate a CBVE procedure and by then comparing the learning effects of use of different versions of the procedure. Three sets of illustration for the same CBVE automotive mechanics assembly procedure were developed: 7 "learner-based" illustrations using the complete model, 10 "developer-based" illustrations using the model without learner data, and 1 "typical" illustration. Random assignments of 173 students to one of three treatments were made; 37 students also performed the assembly procedure. Posttest results indicated few differential effects among the three treatments. Analysis of covariance suggested that students studying the developer-based illustrations performed the assembly significantly better. Results indicated the model seems useful and relevant to designing illustrations, but use of learner input is not necessary. (Appendixes include decision, perception, and memory principles and the three sets of illustrations.) (YLB)