Preceding studies have shown photoplethysmographic (PPG) signal resembles blood pressure (BP) wave and varies. Some investigators also have studied this relationship to explain complex hemodynamic characterization. The purpose of this study is to make a trial of finding arterial BP (ABP) using PPG signal. This new attempt is based on the theory that BP consists of the change of blood volume (BY) and the resistance of vessels. This study proposes a method to estimate BP from PPG signal and points to be considered when we use this method. Therefore we can classify cardiac output (CO) and the blood vessel resistance (VR) by analyzing PPG signal. Signals were obtained from the tails of three healthy dogs and the fingers of six patients who have changes of BP. In the animal test, we first found the correlations between PPG signal and BP, and then we could reconfirm the relationship in the clinical test. PPG signal could be influenced by the pressure (P) that occurred between the index finger and the sensor and the temperature (T) of the interest region, so we examined these influences with six healthy subjects. From the experimental results, we suggest the relationship of the estimation of BP from PPG signal. When PPG signal is normalized by P and T, this PPG signal offers more accurate estimation of BP. This study could be able to provide a new BP measurement system that has not only convenience but also accuracy.