This paper reports the development of critical thinking of urban high school students in the Chinese city of Xi'an. It presents the assessment of the students' two components of critical thinking: dispositions towards critical thinking and critical thinking skills, using the California Critical Thinking Disposition Inventory and the California Critical Thinking Skills Test. Results show that the students' dispositions toward critical thinking are at an average level while their critical thinking skills are very low. This research also shows that there is no significant difference between students from science classes and arts classes in their critical thinking dispositions while science students have a higher level of critical thinking skills. Students from advanced classes have a higher level of the two components of critical thinking than their counterparts from general classes. There are no differences between male students and female students in both components.