The Stellar Initial Mass Function (IMF) is fundamental in determining important galaxy properties such as stellar mass and star formation history. In this talk, I will present findings on the stellar IMF in the center of nearby early-type galaxies (ETGs) in the MASSIVE survey, based on high-quality Magellan/LDSS-3 spectroscopy combined with detailed stellar population synthesis modeling. I will focus on the relations among the IMF, global galaxy properties, and stellar populations. I will present recent progress in measuring the variation of IMF within the ETGs, and discuss the implications for the physical processes driving the IMF variation, as well as the impact on mass measurement.