Low-Ca pyroxenes play an important role in mantle melting, melt-rock reaction, and magma differentiation processes. In order to better understand REE fractionation during adiabatic mantle melting and pyroxenite-derived melt and peridotite interaction, we developed a parameterized model for REE partitioning between low-Ca pyroxene and basaltic melts. Our parameterization is based on the lattice strain model and a compilation of published experimental data, supplemented by a new set of trace element partitioning experiments for low-Ca pyroxenes produced by pyroxenite-derived melt and peridotite interaction. To test the validity of the assumptions and simplifications used in the model development, we compared model-derived partition coefficients with measured partition coefficients for REE between orthopyroxene and clinopyroxene in well-equilibrated peridotite xenoliths. REE partition coefficients in low-Ca pyroxene correlate negatively with temperature and positively with both calcium content on the M2 site and aluminum content on the tetrahedral site of pyroxene. The strong competing effect between temperature and major element compositions of low-Ca pyroxene results in very small variations in REE partition coefficients in orthopyroxene during adiabatic mantle melting when diopside is in the residue. REE partition coefficients in orthopyroxene can be treated as constants at a given mantle potential temperature during decompression melting of lherzolite and diopside-bearing harzburgite. In the absence of diopside, partition coefficients of light REE in orthopyroxene vary significantly, and such variations should be taken into consideration in geochemical modeling of REE fractionation in clinopyroxene-free harzburgite. Application of the parameterized model to low-Ca pyroxenes produced by reaction between pyroxenite-derived melt and peridotite revealed large variations in the calculated REE partition coefficients in the low-Ca pyroxenes. Temperature and composition of starting pyroxenite must be considered when selecting REE partition coefficients for pyroxenite-derived melt and peridotite interaction.