Fracture behavior and mechanical properties of SiC nanofiber (SiCNF) reinforced SiC nanocomposites as influenced by the thickness of amorphous carbon (a-C) coatings are studied via molecular dynamics simulations using Tersoff potential. To simulate the condition that a matrix crack arrives at the interface between matrix and coating, a pre-setting matrix crack is created. Results show that the tensile stress-strain curve of nanocomposites without and/or with thin a-C coatings (e.g., t≤ 0.3 nm) demonstrates an abrupt drop after achieving a maximum value, while nonlinear tails appear in the curves of nanocomposites with thick a-C coatings (e.g., t >2.0 nm). It is demonstrated that the SiCNF is penetrated by the matrix crack when it is uncoated and/or coated by a thin a-C layer (t ≤ 0.3 nm) and the nanocomposite fails in a typical brittle mode; whereas the crack deflection path changes and the SiCNF is pulled out from the matrix when the a-C coatings are thick enough (e.g., 4 nm), showing a different fracture mode in nanocomposites. Compared to nanocomposites without an a-C coating, the tensile strength of nanocomposites with a-C coating of 4.0 nm thickness is about four times higher, and the fracture energy increases around an order of magnitude. Furthermore, the average stress concentration factor for SiCNF in nanocomposites, defined as the ratio of tensile strength of single SiCNF to the average stress of the nanofiber in the composite when it is broken, is extracted and shows a decreasing trend with increasing coating thickness, indicating that a-C coating can therefore be expected to simultaneously enhance the tensile strength and fracture energy of the SiCNF/SiC nanocomposites. This work sheds light on the toughening mechanism in SiCNF/C/SiC nanocomposites where a-C coating plays a significant role, indicating that the toughening mechanism in conventional ceramic matrix composites on a microscale is still valid on a nanoscale. Simulation results suggest that coating thickness in material design is efficient for engineering SiCNF/SiC nanocomposites with high strength and toughness.