An experimental and theoretical study of the fracture strength of compact tension specimens and double cantilever beams made of polymethylmethacrylate (PMMA) under tension was carried out. The critical loads and crack propagation paths for these samples under pure mode I loading conditions differ markedly. Energy-based theoretical model, which allows predicting the instability of the crack growth path, is presented. The theoretical model takes into account both the singular term of stress in front of the crack tip and the first non-singular term known as T-stress. To verify the theoretical model, the experimental results obtained in fracture test on several mode I cracked samples are used. Computer simulation of crack propagation in a geometrically and physically nonlinear formulation has been performed. The experimental data are compared with the calculation results. It is shown that the instability of the crack path substantially depends on the geometry and can be prevented by changing the sample geometry or type of load.