In order to complete an undergraduate degree in Indonesian, college students are mandated to finish their undergraduate thesis (usually called as ‘skripsi’ in Indonesian) in their final year of study. Finishing an undergraduate thesis is known to be a challenging process as one’s ability in problem solving, writing, and conducting research is tested. This study aims to determine the effectiveness of online group Mindfulness Based Stress Reduction (MBSR) in reducing anxiety and negative affect and also increasing positive affect in college students who are currently working on their undergraduate thesis. This study is conducted using a quasi-experimental research design including an experimental (n = 22) and control group (n = 8). The data is collected in 3 time points (pretest, posttest, and 1 month follow-up) using GAD-7 to measure anxiety and PANAS to measure positive and negative affect. It is found that the experimental group had significantly lower score of anxiety (F(2, 56) = 15.90, p = < .001) and negative affect (F (2, 56) = 8.76, p = <0.001) at the follow-up. However, there was no significant mean difference in positive affect. In conclusion, online group MBSR is effective in reducing college students’ anxiety and negative affect during the completion of undergraduate thesis.