Polyamide 6 (PA6) nanocomposites containing cellulose nanocrystals (CNCs) were prepared via a multi-step process consisting of in-situ anionic ring-opening polymerization and subsequent melt extrusion. The effect of surface modification of the CNCs with aminopropyl triethoxysilane (APS) was studied in detail using microscopic, mechanical and rheological techniques and compared with that of the neat CNC nanocomposites. Solid-state Si-29 NMR analysis was used to confirm the interfacial bond formation between the PA6 matrix and surface modified CNCs. SEM images showed that upon surface modification, the morphology of CNCs within the matrix transformed from a fibrillar structure towards more individually dispersed nanocrystals with enhanced dispersion and higher interfacial area. The matrix particle interphase was further investigated using quantitative nanomechanical mapping (QNM) to study the role of interfacial modification on thickness of interphase and development of a broader modulus gradient across the interface. The quality of dispersion and development of the rigid interfacial layer in the modified system resulted in significant improvement in solid-state mechanical properties of the nanocomposites. In addition, melt rheological studies showed significant improvements of melt elasticity and strength in shear and elongational flow in the nanocomposites systems. Published by Elsevier Ltd.