© 2019 Elsevier Inc. The demand for dimensional micro and nano metrology is evident, and with the miniaturization process of microstructures and the trend towards using polymer material, the available technologies are imperative. This paper proposes a quantitative depth evaluation method for microgrooves with width less than the diffraction limit and a measurement system capable of depth detection on polymer materials. The depth of microgrooves can be quantitatively related to the near-field optical phase difference, which cannot be practically observed but can be computed from practical far-field observations by proposed depth evaluation method. The developed measurement system uses low-coherence illumination to reduce the spatial speckle noise and multiple interference noise on transparent polymer materials. Microgrooves (nominal width = 300 nm, Rayleigh Criterion of measurement system = 772 nm) on a transparent polymer surface (thermoplastic COC 5013L molded sample) were measured, and the results were compared with AFM measurements. It is demonstrated that the proposed method is instrumental in evaluating the depth of microgrooves beyond the diffraction limit with an accuracy of less than 10% of the test depth.