In saturated markets cross selling offers a number of benefits to companies. While these benefits are recognized by mobile telecommunications companies, many have not fully embraced the concept in practice. This paper focuses on the prediction of cross selling opportunities and presents an innovative approach to forecast cross selling opportunities more effectively. The proposed approach combines decision tree and association rule to discover cross selling opportunity of a new service, WAP. The analysis is based on a sample of 969,228 customers selected at random from the data warehouse of a local mobile telecommunications vendor in China Mainland. The results showed that this approach can greatly improve the accuracy rate of forecasting and help telecommunications vendors make cross selling policies effectively.