A wearable cuff-less pulse transit time (PTT) based monitoring device is developed for ambulatory blood pressure (BP) monitoring. Ten healthy subjects (aged 27±4 years old) underwent 24-hour ambulatory BP monitoring using 1) a standard brachial cuff-based oscillometric device as reference and 2) the proposed cuff-less PTT measuring system. Raw PTT and BP measurements were linearly interpolated and then smoothed by a low-pass filter to remove aliasing effect caused by the low sampling rate and synchronized. Resampled PTT and BP were assessed for correlation using correlation coefficients and Bland-Altman plots. Our study showed that PTT estimated systolic BP most accurately within 4.8 ± 4.3 mmHg on healthy young subjects during sleep time. We conclude from this study that the proposed cuff-less PTT-based BP monitoring system has potential to be a less intrusive alternative to standard oscillometric method for long-term overnight BP monitoring.