Online therapy is becoming more and more popular among people who seek a newer modality of mental health treatment or cannot travel to a treatment center. The purpose of this research was to explore the effectiveness of online therapy for treating anxiety and depression. We located ten studies (total sample size N = 2,133) from PsycINFO and Google Scholar that reported anxiety and depression measures with pre/post/follow-up online treatment interventions. We coded these articles and recorded the effect size (Cohen’s d). The overall weighted average effect size of the set of studies was quite large (d = 0.77). For participants 30-35 years old, the weighted average effect size was even larger (d = 1.48). On-line therapy’s effects are not just short-lived; they were effective even at 1-3 months follow-up (d = 1.63). Findings from this study provide insight into the effectiveness of internet-based therapy on people with anxiety and depression.