The structural and electrical properties of CaB4 under high pressure have been studied by angle dispersive X-ray diffraction, in situ Hall effect measurement, and first-principles calculations. An abnormal change of the c/a ratio was observed around 12 GPa in the experiments and backed by theory calculation, indicating an isostructural phase transition. Unlike other materials, the analysis of the electronic and phonon band structure doesn't reveal an electronic topological transition or phonon softening supporting this phenomenon. While the subtle changes in the electron band dispersions at the Fermi surface shown by the measurements of the resistivity, Hall coefficient, carrier concentration, mobility, and temperature-dependent resistivity can explain the anomaly in the c/a ratio. The study of electrical transport properties provides strong support for the occurrence of the isostructural phase transition in CaB4.