Mobile number portability (MNP) means that customers using mobile service can retain their telephone numbers when they change operators. Mobile carriers are concerned about which factors influence willingness to churn of customers. This study examined these concerns through three steps. The first is checking whether willingness to churn between the different situations, without MNP and with MNP differs. The second is to figure out how different factors influence willingness to churn in those situations. Finally, a classification model is proposed to categorize the subscribers who have willingness to churn or not. This study shows that the difference of willingness to churn before and after MNP. The results show that quality and price are the most influential factors in both cases. Inconvenience of changing numbers is additional factor in case of the unavailability of MNP. The results have implications particularly that mobile carriers should consider their quality of service and price more under the new situation.