Epitaxial CaTiO3 thin films with thicknesses between 4 and 60 nm were grown on (001)-oriented SrTiO3 substrates by liquid-delivery spin metal–organic vapor phase epitaxy, imposing an in-plane tensile lattice mismatch of approximately 2.2%. Atomic force microscopy, high-resolution X-ray diffraction, and reciprocal space mappings reveal that CaTiO3 films remain globally coherently strained up to 11 nm, while macroscopic plastic relaxation sets in between 11 and 16 nm, defining the experimental critical thickness of the CaTiO3/SrTiO3 system. Scanning transmission electron microscopy reveals a thickness-dependent evolution of strain relaxation pathways: the 11 nm film exhibits predominantly uniform contrast consistent with coherent epitaxy, with only rare cracks or misfit dislocations observed at lateral length scales exceeding several micrometers; the 16 nm film displays partial relaxation via cracking and anti-phase boundaries at length scales of several hundred nanometers; and the 60 nm film contains a high density of cracks, misfit dislocations, and pervasive anti-phase boundaries. These extended defects are known to generate pronounced local strain gradients. The results provide a structural framework for understanding defect-mediated strain relaxation and strain-gradient effects in CaTiO3/SrTiO3 heterostructures.