Deep silicon trench technology is essential for fabricating vertical microelectronic devices, which is gaining increasing popularity due to the ever shrinking device dimensions. The control of critical dimensions (CD) and sidewall profiles of silicon trenches is critical to the device performance and production yield, posing a need for in-line metrology for process control. The widely used cross-section scanning electron microscopy (X-SEM) is not suitable for in-line monitoring because of its destructive nature and long turn-around measurement time. Alternative techniques, for example, atomic force microscopy (AFM), are constrained by the aspect ratio of the trench and limited by throughput. In this paper we report results of trench profile measurement using a non-destructive Optical Critical Dimension (OCD) technique. In this technique, trench profile parameters are measured using normal incidence spectroscopic ellipsometry. We studied various silicon trenches with depths between 1 and 2.5 um and pitches from 1 to 4 um. Spectral simulation and profile comparison between OCD and X-SEM/AFM reveal that OCD has great sensitivity to deep silicon trench profile features (CD, trench depth and side wall angle), providing a viable metrology solution for real time process control.