To investigate the fracture behavior of fly ash geopolymer concrete, three point bending tests of fly ash geopolymer concrete samples were carried out. The digital image correlation(DIC) method was proposed to measure the crack mouth opening displacements(CMOD), crack tip opening displacements(CTOD), as well as the crack extension length. For the purpose of cross validation, the extensometer was also employed for crack measurement. The results show that the fracture process can be separated into four different stages, i.e. crack initiation stage, crack stable extension stage, crack sub steady extension stage, and crack unstable extension stage. The values of CMOD measured by the two methods matched very well with each other, which helps to validate the effectiveness of DIC method. In addition, the values of CTOD measured by DIC show good agreement with the results calculated from the elastic equivalent crack method. However, the crack extension length measured by DIC is much larger than the calculated results. It is attributed to the formation of fracture process zone in the front of the crack tip, which has also been observed in cement concrete.