Three-dimensional (3D) cell culture models are developed as a promising platform to screen anticancer therapeutics and treatments. However, current imaging techniques cannot provide 3D structures of tumor spheroids in situ. In this study, we employed label-free and noninvasive optical coherence tomography (OCT) for imaging and quantifying the 3D structures of tumor spheroids. We imaged ovarian cancer spheroids with OVCAR-8 cell line over a period of 10 days with 5,000 and 50,000 initial cell numbers. We successfully reconstructed the 3D necrotic regions via label-free intrinsic scattering attenuation contrast and evaluated the effect of Cisplatin treatment on tumor spheroids.