Thermal comfort is an important and influential indicator of human quality of life and function. In general, thermal comfort index indicates the level of balance in the sympathetic nervous system. When the inlet and outlet heat levels are equal and there i s no need for the sympathetic nervous system intervention to adjust the temperature, the body feels thermally comfortable. Nowadays, neglecting this topic in designing and manufacturing high performance sport , military, firefighting clothing means ineffici ent product. Also, Attention to This Index Have Led t o A Fundamental Change in Modern a nd Intelligent Technologies Such a s Electronic Clothing With The Ability t o Manage Body Temperature a nd Humidity . The purpose of this study for first step is to identify thermal comfort and find a practical method for measuring it. In the second step, the construction of a thermal comfort index sensor and analyzing its results under real conditions. In this study, frequency analysis of heart rate variability (HRV) was used as one of the newest and most accurate methods of thermal comfort measurement. It measures the ratio of the function of the sympathetic nervous system (which regulates body temperature) to the function of the vagous nervous system. The meth od used to measure this index is to measure the low frequency power (0.04 - 0.15 Hz) to the high frequency power (0.15 - 0.4 Hz) and report it as an LF / HF index. After An exercise activity, the LF / HF index is influenced by the heart rate and also the heart rate is influenced b y the body temperature. Therefore the proper function of clothing in exchanging body heat with the environment , accelerates t he process of reaching the body balance. In this case, the feeling of comfort increases with decreasing activi ty duration of the sympathetic nervous system . Since the measurement of thermal comfort index over a long period of time while it is not intended to cause disturbance, in this study, contrary to the conventional methods of measuring heart rate, which is based on the cardiac activity recorder, a long - term wearable sensor based on piezoelectric effect was used to measure heart rate. I n the nuclear fabrication of this sensor, PVDF piezoelectric nanofibers were used. In order to fabricate the sensor with optimum sensitivity, single nozzle electro spinning with optimized parameters was used. Thus, by producing highly oriented nanofibers co ntaining the beta phase, the sensor has a minimum sensitivity of 5 N, which is appropriate for heart rate measurement. by connection of sensor core to the electronic circuit of signal conditioning and processing, final sensor built and used in field test s t o detect thermal comfort indices . 3 clothing s of different genders including: Co o lm a x, cotton and polyester used in field tests . The LF / HF index was measured for each. The amount of comfort obtained for polyester clothing was significantly lower than the comfort value obtained from C o olmax and cotton clothing. Also, the LF / HF index values were significantly correlated with participants' qualitative evaluation results.