In this study, research using guided inquiry was conducted to improve students' ability to think critically. By improving and making the worksheets the students of this research are expected to make it easier to understand a lesson. The development step in this study is to conduct a preliminary study as the first stage, then pre-product planning and development, after which the pretest, product prerevision and field test are carried out. Data collected in this study was carried out by observation, documentation, interviews, questionnaires and tests. In addition, student worksheets are validated based on media and material, which will be given revisions based on suggestions from the media, matrices, and experts. The participants of this study were students Junior High School of SMP Negeri 30 Bandar Lampung, Indonesia. The final results in this study, showed that the development of guided inquiry based worksheets was able to improve students' critical thinking power, which was obtained from the results of good category values, namely 71.07% on average students in mathematical critical thinking.