This study examined the effects of the introductory organic chemistry online homework on first year pre-service science teachers' learning achievements. The online homework was created using a web-based Google form in order to enhance the pre-service science teachers' learning achievements. The steps for constructing online homework were approximately described. The participants were 76 first year pre-service science teachers at Suan Sunandha Rajabhat University (SSRU). A quasi-experimental pretest-posttest design was performed in this study. The research instruments were pre- and post-tests and online homework relevant to the introductory organic chemistry. The findings unfolded that the pre-service science teachers' performance significantly improved after the implemented treatment and the online homework was an effective learning aid for improving the achievements of introductory organic chemistry learning. The correlations between online homework score and the various course elements were investigated. The moderate and statistically significant correlations were found between the online homework score of the students and the normalized gain and between the online homework score and their post-test achievements. Hence, the score results of the online homework were considered as a predictor of student test scores. A statistically significant correlation between their achievements in introductory organic chemistry chapter and their overall grades was found. In term of learning behavior, this implied that students' learning habits improved when they completed their online homework. This study also proposes that introductory organic chemistry is the hardest chapter in this general chemistry course sequence at SSRU.