Measuring carbon-14 (¹⁴C) activity in the material of biological origin provides the age of that material, and also indicates the radioactive contamination due to radiocarbon in existing organisms, plants, etc. Several techniques comprising direct and indirect methods are used in dating employing ¹⁴C. This study introduced a new method for calculating the ¹⁴C activity using Thick Gas Electron Multiplier (THGEM) detector in Self Quenching Streamer (SQS) mode. In this method, by placing the ¹⁴C contaminated sample in front of the THGEM detector, the correlation between the ¹⁴C activity and the number of SQS light in THGEM holes is obtained via both the Monte Carlo simulation and measurement. The obtained results show that the THGEM detector is suitable to determine the ¹⁴C activity in existing organisms and plants. The proposed THGEM detector removes the necessity of expensive electronic amplifiers in conventional ¹⁴C detectors. It is expected that the THGEM can be efficiently used in ¹⁴C dating and determine the radioactive contamination caused by radiocarbon in plants and the environment.