Security breaches have been observed in different dimensions in mobile payment system. The violation of user's privacy is a common phenomenon in mobile payment transactions. This study presents an improved security scheme for a mobile payment system using elliptic curve cryptography over a binary field with International Mobile Equipment Identity to ensure higher security. The scheme uses a payment gateway for registration and maps all input text to elliptic curve points using ASCII values. Payment details are stored on the gateway, which is encrypted but decrypted only with merchant's decryption key. The proposed scheme was evaluated in terms of key size, security strength, computational power, memory capacity, encryption and decryption time and mobile phone battery. The result shows that the scheme provides integrity, confidentiality and privacy. The result also shows that the proposed scheme is time-efficient and computationally inexpensive for resourceconstrained environment like mobile payment system.