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Development of 21st century skills becomes essential life skills in supporting individuals to adapt well to 21st century life. One of the 21st century skills that needs is critical thinking skills. Development of student skills can be done through learning and appropriate assessments. The application of assessment for learning is an effort that can be done to develop student skills. The feedback provided can help students to plan and improve their learning strategies. However, the application of assessments in the learning process is often not optimal. Therefore, it is necessary to do research on the application of assessment for learning to improve students critical thinking skills. This study uses quasy experiment method with the pretest posttest control group design. Assessment for learning through written feedback is applied to experimental class, while the control class is given of conventional assessment. Indicators of critical thinking skills in this study were developed based on the Partnership for 21st Century Skills. The results showed that the application of assessment for learning through written feedback can improve students critical thinking skills. This is indicated by the experimental class average N-gain value is 0.82 in Pteridophyta and 0,87 in Bryophyta subchapter.