The improvement in the hardness of Sn-3.0Ag-0.5Cu solder alloy reinforced with 1.0 wt % TiO2 nanoparticles was evaluated by nanoindentation. A specific indentation array was performed on four different horizontal cross sections of the composite solder with different heights and diameters, in order to verify the mixing homogeneity of TiO2 nanoparticles within the Sn-3.0Ag-0.5Cu solder paste during the ball milling process. The phase analysis indicated successful blending of the Sn-3.0Ag-0.5Cu with the TiO2 nanoparticles. According the scanning electron microscopy micrographs, presence of the TiO2 nanoparticles reduced the size of the Cu6Sn5 and Ag3Sn intermetallic compound phases. Incorporation of the 1.0 wt % TiO2 nanoparticles improved the hardness values up to 26.2% than that of pure SAC305. The hardness values increased gradually from the top cross sections towards adjacent to the solder/substrate interface. The mechanism of the hardness improvement attained by the TiO2 nanoparticles addition were also investigated on the horizontal cross sections of the samples.