The aim of this study was to investigate the effect of learning diaries on developing Critical writing skills for Al Azhar preparatory school pupils. An quasi experimental method was adopted. Sample members of the study were 80 students, (N= 80). They were randomly chosen and divided into two groups: the experimental group and the control group after being  sure  that  they  were  equivalent. A critical writing test was used to measure the critical writing skills for the Al Azhar preparatory school pupils of the sample members. Based on the students' mean scores of the test, results showed that the performance level of EDSs' writing was low. The  experimental  group students  were  trained  in  critical  writing skills  through  the  use  of  diaries learning, while the regular instruction was used with the control group. To achieve the aims of the study, the researcher designed a critical writing skills pre/ post test to measure the students' critical writing skills. It was administered to them before and after the experiment.  The  mean  scores  of  the  pre  /  post  administrations  of  the  test were treated statistically. Results also showed that there is a statistically significant difference between the post-test administrations mean scores of the experimental and the control groups in critical writing skills in favor of the experimental group."