The characteristics of high-power, high frequency transistors based on GaN/AlGaN are affected by the presence of strain, which can be unintentionally present due to heteroepitaxial growth or can be intentionally introduced. The strains cause changes to the band dispersion and band gaps of the materials, which can alter device performance. Previous studies have focused on the effects of strain on the band gap of AlN and GaN, but relatively little work has been devoted to investigating the effects on the effective mass. Using first principles calculations based on density functional theory (DFT), we have conducted a systematic study of the effects of biaxial and hydrostatic stress, and the resulting strain, on the band properties of AlN and GaN. In particular we analyze the changes in band gap and electron effective masses with strain. Electron effective masses were determined both parallel and perpendicular to the <I>c</I> direction in hydrostatic and biaxial stress configurations. We find that for small strains (ε<SUB>xx</SUB>= ε<SUB>yy</SUB>= ± 0.01), the band gap and effective mass generally decrease linearly with tensile strain, as expected from nondegenerate k.p theory. A notable exception occurs in AlN in the direction parallel to the <I>c</I> direction; in this case, we observe a linear increase in effective mass with tensile strain, which can be explained by a four-band Kane model analysis using k.p theory. This work was supported by NSF and by the UCSB Solid State Lighting and Energy Center.