Positive psychology intervention is mediation that aims to promote quality of life and well-being. Current research integrating positive psychology with the Internet is called online positive psychology (OPP) which promotes and prevents mental health problems, improves well-being, and reduces depression. This experimental research aimed 1) to compare the psychological well-being of the experimental group that received online positive psychology intervention in the phase of pre-test, post-test, and follow up and 2) to compare the psychological well-being between the experimental group and the controlled group. The subjects were 24 undergraduate students from Mahasarakham University, Thailand, selected by purposive sampling. Thereafter, the subjects were equally divided into experimental and controlled groups. Measures used in this study were as follows: 1) the online positive psychology intervention to improve psychological well-being and 2) the scale of psychological well-being based on Ryff's psychological well-being. The statistics used in the data analysis were the Friedman Test, Wilcoxon signed-rank test, and the Mann Whitney U Test. The results of the pre-test and the post-test showed that the mean scores of psychological well-being of the experimental group were significantly different at 0.05 levels. Additionally, the mean scores of psychological well-being between the experimental group and the controlled group in the phases of post-test and follow-up were significantly different at 0.05 levels. The online positive psychology intervention was effective in increasing the psychological well-being of undergraduate students.