Testing methodologies integrating wedge-loaded double cantilever beam (WDCB) methods with digital image correlation (DIC) are promising for measuring the Mode I interlaminar fracture toughness of fiber-reinforced composites owing to the ability to bypass the need for direct force measurement. However, such testing approaches tend to rely on an accurate evaluation of the crack length data using DIC, which still lacks systematic studies. The present research investigated the application of DIC for measuring the Mode I fracture toughness of a carbon-fiber-reinforced composite material tested using the WDCB method. A DIC procedure was adapted to measure the crack length history, and the proposed DIC approach was validated by comparing with a beam theory–based method found in the literature. Emphasis was given on analyzing the effects of three typical DIC parameters (the number of virtual extensometers, the height of the virtual extensometers, and the strain-based threshold used for crack tip identification) on the derived crack length history. For efficiency, the Crack Length Gauge tool available in the Imetrum Video Gauge software, which has the option to adjust the three DIC parameters and generate results equivalent to those of the proposed DIC procedure, was employed to perform the DIC-based sensitivity analysis. The results highlighted that the crack length history was independent of the height of virtual extensometers but was affected by the number of virtual extensometers and the threshold value used. To obtain a reasonable measurement of crack propagation, it was recommended that the number of virtual extensometers should be sufficiently large and the threshold value should be sufficiently small (no larger than the failure strain in the thickness direction).