The work aims to present an ultra-low power Electrocardiogram (ECG) on a chip with an integrated Fuzzy Decision Making (FDM) chip for Wireless Body Sensor Networks (WBSN) applications. The developed device is portable, wearable, long battery life, and small in size. The device comprises two designed chips, ECG System-on-Chip and Fuzzy Decision Maker chip. The ECG on-chip contains an analog front end circuit and a 12-bit SAR ADC for signal conditioning, a QRS detector, and relevant control circuitry and interfaces for processing. The analog ECG front-end circuits precisely measure and digitize the raw ECG signal. The QRS complex with a sampling frequency of 256 Hz is extracted after filtering. The extracted QRS details are sent to the decision maker chip, where abnormalities/anomalies in patient’s health are detected and an alert signal is sent to the patient via wireless communication protocol. The patient’s ECG data is wirelessly transmitted to a PC, using ZigBee or a mobile phone. The chip is prototyped and employed in a standard 0.35 µm CMOS process. The operating voltage of Static RAM and digital circuits and analog core circuits are 3.3 V and 1 V, respectively. The total area of the device is about 6 $$ \text{cm}^{\text{2}} $$ and consumes about 8.5 µW. Small size and low power consumption show the effectiveness of the proposed design, suitable for wireless wearable ECG monitoring devices.