1,050 teenagers in Bangkok Metropolitan Administration (areas) were interviewed regarding their mobile phone usage. The study showed that males and females differed in their mobile phone use. Males (89.7%) selected all the applications available for their mobile phones. Between the two male groups, 95.6% of the high school males (aged 15-17 years) spent more than 4 hours per day for entertainment and social networking, such as facebook and twitter, and their mobile phone expenses were more than 600 baht per month, whereas, 86.9% of university males (age 18-20 years) spent 1-2 hours per day for education and reading the news and their mobile phone expenses were about 600 baht per month. For females, 91.2% of them selected the full application package for their mobile phones. Between the two female groups, 98.4% of the high school females (age 15-17 years) spent more than 4 hours per day for entertainment and social network, such as facebook and twitter and their expense were 1,001-1,500 baht per month, whereas (89.6%) of university females (age 18-20 years) spent 2-4 hours per day for education, social networking, such as facebook and relaxing from stress, with expenses of 1,001-1,500 baht per month. It can be concluded from the result of the assessment that decision makers should invest in the mobile phone businesses focusing on the main target group of female teenagers because they spend a lot of money on mobile phones, both the equipment and usage.