The heart is one of the most vital organs in the human body that pumps blood. Therefore, it is very important to monitor the condition of the heart. The condition of a human heart can be diagnosed by measuring the heart rate frequency (beats per minute). We propose a design of a portable electrocardiography (ECG) based on capacitive electrodes for heart rate monitoring. The device consists of electrodes attached to a plastic sheet which is placed on a mattress bed under a patient's back. Before being processed by Arduino UNO microcontroller, the output signal of the electrodes is processed using signal-conditioning circuit consisting of instrumentation-amplifier circuit, filter, and non-inverting amplifier. The signal processing results obtained from the conditioning circuit are displayed on the LCD screen. The test results show that the QRS complex signal can be clearly detected. Various thickness of the patient cloth also produces different values. With this factor taken into account, cottons with varying thickness (from 0.2mm up to 0.8mm) are used to measure the heart rate signals. Measurements with varied cotton thickness shows that the thicker the clothing material is, the weaker and more fluctuative the cardiac signal read by the capacitive ECG design will be. To test the accuracy of the proposed design, the measurement result is compared to ECG PM 10 which is a commonly used device for heart rate monitoring. It shows that experiment on different subjects result in an average error of 0.435%. Based on the Association for the Advancement of Medical Instrumentation (AAMI)'s standard of maximum error (5%), the error rate of the proposed device is still within the range of tolerance.