Based on writing weekly academic journals and on Bloom's (1984) taxonomy of cognitive critical thinking skills, this article reports on a quasi-experiment where journal writing was an additional task to an academic writing course. The experiment was carried out with first year university students (semester two) in one of the Egyptian private universities. Sixteen intermediate students represented the experimental group where weekly journals were a requirement; another seven students of the same level represented the control group that was taught exactly the same course and by the same instructor of the experimental group but with no journal writing. Pre and posttests were used to measure students' critical thinking skills before and after the experiment respectively. Measuring critical thinking skills using the available critical thinking assessment tests was not suitable for the purpose of the experiment since the intended skills were those associated with Bloom's taxonomy not just reasoning and logical thinking. Therefore, the researcher, following Bloom's taxonomy of cognition and inspired by the available critical thinking assessment tests, designed her own critical thinking assessment tool. This tool is an attempt to make use of any available materials to create one's own instrument that could serve in the learning process. The newly modified tool was used in the pre and posttests. Results indicated that journal writing had contributed strongly to the development of the cognitive critical thinking skills of the experimental group, which validates the new assessment tool and proposes a new technique of developing critical thinking skills in Egypt.