In this study a heart-beat-interval counterbased a low-power PPG microsystem is proposed for accurate assessment of the mental stress. The designed microsystem incorporates a new low power PPG sensing readout with a Time-to-digital converter for the long-time continuous heart-beat-interval estimation. Further the analog front-end circuit is implemented in the integrated chip having an area of 1.4 mm2 and fabricated using the TSMC 0.18 µm process. Measured linear sensing range of the designed readout is 20 nA to 110 µA. With the 1.8 V standard supply, measurement results show that the power consumption of the PPG readout circuit is 52.2 µW, while the total measured power consumption of the designed chip is 100.2 µW. To evaluate the performance of the proposed microsystem in mental stress assessment, the designed circuit is integrated with OLED-OPD sensor and then applied to the wrist of two healthy subjects under different stressors, (e.g., laughing, solving a mathematical problem, hearing loud audio/sound, and moving neck). The statistical analysis of the detected PPG signal and measured on-chip-heart rate interval in time domain shows that the mean value of peak-to-peak interval, entropy, and stress-induced vascular index increases during the stress. In addition, in the frequency domain analysis of the heart rate variability (HRV) shows that the ratio of the low frequency component to the high frequency component is increased during the stress. Thus, the indices of HRV measured directly from the designed readout system can serve effectively as indication of heart rate variability and mental stress.