The negative thermal expansion (NTE) and correlated magnetic and electrical transport properties of Mn3GaxSi1-xN were investigated. For pure Mn3GaN, there is a large NTE effect corresponding to the antiferromagnetic to paramagnetic transition. Very interestingly, when partial Ga was replaced by Si, the NTE properties around the magnetic transition were changed. The NTE temperature range was broadened to delta T=148 K for Mn3Ga0.75Si0.25N and the linear thermal expansion coefficient was estimated as beta=-1.4 x 10-5 K-1 (272-420 K). Accordingly, the resistivity also showed a decrease from 327 to 395 K with temperature. With a further increasing Si content to x=0.5, the magnetic transition still occured, but the NTE effect did not appear. After careful observation, an anomaly was found at around 350 K in a-T, -T, and DSC curves of Mn3Ga0.5Si0.5N, respectively. This phenomenon strongly implies the close correlation among lattice, spin, and charge in this series materials.