The effect of post-growth annealing treatment of zinc oxide (ZnO) nanorods on the electrical properties of their heterojunction diodes (HJDs) is investigated. ZnO nanorods are synthesized by the low-temperature aqueous solution growth technique and annealed at temperatures of 400 and 600 degrees C. The as-grown and annealed nanorods are studied by scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. Electrical characterization of the ZnO/Si heterojunction diode is done by current-voltage (I-V) and capacitance-voltage (C-V) measurements at room temperature. The barrier height (phi(B)), ideality factor (n), doping concentration and density of interface states (NSS) are extracted. All HJDs exhibited a nonlinear behavior with rectification factors of 23, 1,596 and 309 at +/- 5 V for the as-grown, 400 and 600 degrees C-annealed nanorod HJDs, respectively. Barrier heights of 0.81 and 0.63 V are obtained for HJDs of 400 and 600 degrees C-annealed nanorods, respectively. The energy distribution of the interface state density has been investigated and found to be in the range 0.70 x 10(10) to 1.05 x 10(12) eV/cm(2) below the conduction band from E-C = 0.03 to E-C = 0.58 eV. The highest density of interface states is observed in HJDs of 600 degrees C annealed nanorods. Overall improved behavior is observed for the heterojunctions diodes of 400 degrees C-annealed ZnO nanorods.