Understanding customers' behavior normally helps planning better marketing strategies, which could lead to an increase in market share and profitability. Loyal customers are always considered as the most important assets for any firm. This paper presents a survey to detect factors influencing the formation of loyal customers' mental image. The proposed study uses factor analysis to determine these factors by designing a questionnaire and distributing among some loyal customers who do banking business in Bank Melli Iran located in city of Tehran, Iran. The results indicate that there were eight important factors influencing customer loyalty including social status, business identity, brand strength, the contract role, organizational benefit, consumer rights, organizational image and supporting power.