Pulse transit time (PTT) is usually determined as the time interval between the peak of the electrocardiogram (ECG) R wave and a characteristic point of the peripheral pulse measured by photoplethysmography. However, it was found that the photoplethysmogram (PPG) is affected by the contact force between the photoplethysmographic sensor and the measurement site, i.e. finger. Therefore, we hypothesized that the PTT may be altered by the applied sensor contact force. An ECG and PPG were recorded simultaneously from 30 healthy subjects using 12 magnitudes of contact force (0.1 N-1.8 N). Three characteristic points of the PPG were selected to determine the PTT, including the points near the foot (PTT1), on the systole phase (PTT2) and near the peak (PTT3). Both PTT1 and PTT2 increased significantly (p = 0.014 for PTT1 and p = 0.038 for PTT2) with the sensor contact force until the transmural force reached -0.1 N. With further increase in the contact force, they kept at an almost constant level. Such a changing trend was not found in PTT3. The results of this study suggest that the applied sensor contact force should be carefully controlled in PTT measurement to avoid diminishing its value as a diagnostic tool.