CriticalThinkingisa processofanalyzingindividualexperiences inasituationfor decision making.This skill isessential fornursing processand clinical practices.Thus, teachingstrategies are needed for enhancing critical thinking in nursing students.This study aimed to compare the pre-post critical thinking skill test among nursing students by applying reflective thinking strategy (reflective journal) in psychiatric clinical practices and to find if a reflective thinking strategy would improve nursing students’ critical thinking skills. A two- group experimental study with a pre-post critical thinking test was conducted. The sample were 3rd year nursing students in Boromrajonani college of nursing Chiang Mai Thailand. The participants were systematic divided by GPA intoaninterventiongroup (n=24)assigned to doreflectivejournaling after their clinical nursing activities and a control group (n= 24) without reflective journaling, during their psychiatric clinical practices for 4 weeks. The reflective journaling forms based on Gibbs’s Reflective Cycle and the critical thinking test by Sriprai Chaiya with validity of 7.6 were used.The score of pre-post critical thinking test were compared within groups by pair t-testand the postcritical thinkingtest scoreofcontroland interventiongroups werecompared by Independent t – test Researchfindingas follows :1.Thescoreof pre-postcritical thinking skillof bothcontrol and interventiongroups were in moderatelevel ( x _ = 49.2 + 1.33 )and Inferenceskill was the lowest in the 5 factors of critical thinking skill. 2.The post-test critical thinking score was significant higher than pre-test score in the intervention group ( t = 0.046 , P< 0.05) but no statistically difference in the control group( t = 0.669 , P> 0.05). 3. There is no significant difference of post-test critical thinking score between control and intervention groups ( t = 0.654 , P> 0.05). Conclusionand suggestionof this study, It waseffectivetoimplement reflectivethinking strategy in the clinical practice to improve the critical skills in nursing students. Further teaching strategies should be considered to promote the inference critical thinking skill in nursing students.