Critical thinking skills are one of the important skills in dealing with the demands and challenges of the 21st century. This study aims to determine the effectiveness of Stimulating Higher-Order-Thinking Skills (Stim-HOTs) model in improving students' critical-thinking skills on cell metabolism topic. This research is quasiexperimental research with pretest-posttest control group design in one high school in Surakarta, Indonesia in 2017/2018 academic year with one class as the experimental class by using Stim-HOTs model while the other class as the existing class that uses discovery learning model. The data were obtained by using criticalthinking skill test instrument in the form of essay that has been through expert validations, namely the validation from the education and evaluation expert, biology expert, and the certified high school Biology teacher, as well as the empirical validation through try-out. The result showed that the average score of critical-thinking skill gain score calculation in the experimental class was higher by 0.66 compared to the existing class of 0.51.