Abstract Teacher Taskforce-based Training Model to Improve the Ability to Develop Classbased Assessment Instruments. This R amp D project was aimed at demonstrating the most effective model out of three training models-Teachers Working Group (KKG)-Practice-Reflection (KPR) Modelling-Practice-Reflection (MPR) and Telling-Practice-Reflection (CPR). The first model was developed based on orientation results. This model was validated by educational ezperts and practitioners and was tried-out so as to result in a model which was more appropriate for primary schools. In the stage of semi-summative evaluation for the final design an ezperiment was conducted to identify the most effective model. The ezperiment employed a factorial design. The findings show that MPR was the most effective model. This model was perceived as the one which was beneficial to improve the teachers capability in designing the instruments of classroom-based assessment for science teaching. In addition the model could improve the teachers knowledge of classroom-based assessment and could help them design better classrom-based assessment instruments. The second most effective model was KPR. Even though this model was perceived as less beneficial than MPR with training carried out twice it could improve the teachers capability in designing the instruments of classroom-based assessment as effective as that of MPR model.