Recently reported very high thermal conductivities in cubic boron arsenide (BAs) and boron phosphide (BP) crystals could potentially provide a revolutionary solution in the thermal management of high power density devices. To fully facilitate such an application, the compatible coefficient of thermal expansion (CTE) between the heat spreader and the device substrate, in order to minimize the thermal stress, needs to be considered. Here, we report our experimental CTE studies of BAs and BP in the temperature range from 100 K to 1150 K, through a combination of X-ray single crystal diffraction and neutron powder diffraction. We demonstrated that the room temperature CTEs, 3.6 ± 0.15 × 10−6/K for BAs and 3.2 ± 0.2 × 10−6/K for BP, are more compatible with most of the semiconductors including Si and GaAs, in comparison with diamond, and thus could be better candidates for the future heat spreader materials in power electronic devices.