A model for the single-edge notched-beam (SENB) test is presented which allows the load-displacement curve of fiber-reinforced ceramic-matrix composite (CMC) specimens to be calculated using the crack-opening function. This function considers the effects of the main parameters of the composite components that govern the fracture of the composite (e.g., fiber strength and interfacial properties). Then, in an experimental study, the SENB curves of an oxide-oxide composite are measured in the as-processed state and after exposure at elevated temperatures. Comparison between the measured and calculated SENB curves indicates that the characteristic loss of the maximum load in the SENB curves due to the annealing is caused by the corresponding reduction of the in situ fiber strength. This method then is proven to be an excellent tool to evaluate the actual fiber strength in CMCs, which is, otherwise, barely measurable. It will be shown that the SENB test is very much qualified for a proper investigation of the relationship between in situ fiber strength and composite strength (the maximum SENB load), because of the limited influence of other parameters (Weibull modulus of the fiber strength and the friction coefficient) on the maximum SENB load.