This project is aimed to design a security system for banks using wireless technology GSM. A GSM modem provides the communication interface. It transports device protocols transparently over the network through a serial interface. A GSM modem is a wireless modem that works with a GSM wireless network. This GSM Modem can accept any GSM network operator SIM card and act just like a mobile phone with its own unique phone number. Advantage of using this modem will be that you can use its RS232 port to communicate and develop embedded applications. Applications like SMS Control, data transfer, remote control and logging can be developed easily. The modem can either be connected to PC serial port directly or to any microcontroller. This project is designed on 8051 micro controller, which is interfaced with a GSM modem to send the message about the lost card details to the user. Here how it works: In this project we interface the ATM machine to GSM modem, Whenever any user loses his ATM card, he puts a complaint in the bank just by sending message, When this lost card if placed in the ATM center the card details will be immediately sent to the user and near by police station, Therefore one gets maximum security to their ATM card by monitoring it continuously. The controller will send the SMS through GSM modem, via serial communication protocol RS 232. An LCD is interfaced to the project to display the status of the system. This project uses regulated 5V, 500mA power supply. 7805 three terminal voltage regulator is used for voltage regulation. Bridge type full wave rectifier is used to rectify the ac output of secondary of 230/12V step down transformer.