This study investigated the relationship of internet addiction with physical and mental health, and academic achievement of junior high school students. A total of 6,265 students from four junior high schools in Haulien City, age ranging from 12 to 15 years old, were selected as our population. Using a stratified sampling, a structured questionnaire was delivered to 1,145 students, of which 1,139 were valid, recovery rate 99.48%. Data were analyzed using SPSS11.5 statistical analysis software. The results showed that as the level of internet addiction increases, the level of physical and mental health decreases. The level of internet addiction also exhibited a negative correlation with the scholastic ranking and average grade. Parents and school should express care and provide counseling aid junior high school students to use internet appropriately, and encourage them to develop other interests and activities in order to reduce the potential for over-indulgence in internet.