We present a new three-dimensional seismic velocity model of the crustal and upper mantle structure for Mauna Loa and Kilauea volcanoes in Hawaii. Our model is derived from the first-arrival times of the compressional and shear waves from about 53,000 events on and near the Island of Hawaii between 1992 and 2009 recorded by the Hawaiian Volcano Observatory stations. The V_p model generally agrees with previous studies, showing high-velocity anomalies near the calderas and rift zones and low-velocity anomalies in the fault systems. The most significant difference from previous models is in V_p/V_s structure. The high-V_p and high-V_p/V_s anomalies below Mauna Loa caldera are interpreted as mafic magmatic cumulates. The observed low-V_p and high-V_p/V_s bodies in the Kaoiki seismic zone between 5 and 15 km depth are attributed to the underlying volcaniclastic sediments. The high-V_p and moderate- to low-V_p/V_s anomalies beneath Kilauea caldera can be explained by a combination of different mafic compositions, likely to be olivine-rich gabbro and dunite. The systematically low-V_p and low-V_p/V_s bodies in the southeast flank of Kilauea may be caused by the presence of volatiles. Another difference between this study and previous ones is the improved V_p model resolution in deeper layers, owing to the inclusion of events with large epicentral distances. The new velocity model is used to relocate the seismicity of Mauna Loa and Kilauea for improved absolute locations and ultimately to develop a high-precision earthquake catalog using waveform cross-correlation data.