In situ TiB whisker and TiB2 particle hybrid reinforced copper matrix composites were fabricated by vacuum induction melting. The phase constitution of the composites varies with their chemical composition. Thermodynamic and kinetic assessments were used to analyze the phase selection of titanium borides during solidification. The precipitations of TiB whisker and TiB2 particle from Cu-Ti-B liquids are both thermodynamically favored, and the phase selection is thus controlled by the competition of nucleation and growth kinetics between the two titanium borides. For the case of the three liquids studied in this work, the nucleation conditions are satisfied except for the formation of TiB in Cu-2at.%Ti-4at.%B liquid. With the change of chemical composition, the growth rate is altered rapidly for TiB, but remains nearly unchanged for TiB2.