The transport properties of Zn0.88-xMgxMn0.12O/ZnO modulation-doped heterostructures (x≤0.15) were investigated. The heterostructures were fabricated on ZnO single-crystal substrates by a pulsed laser deposition system. Atomic force microscope observation and X-ray diffraction analysis suggested that Zn0.88-xMgxMn0.12O layers have atomically flat surface and excellent crystallinity. The results of Hall measurement for Zn0.88-xMgxMn0.12O/ZnO modulation-doped heterostructure with x=0.075 revealed that the carrier concentration and the electron mobility were 5.1×1012cm-2 and 800 cm2/Vs at 10 K, respectively, suggesting that the carrier confinement effect exits at the heterointerface between Zn0.88-xMgxMn0.12O barrier layer and ZnO channel layer. In the magnetoresistance (MR) measurement at 1.85 K, a positive MR behavior was observed below 0.5 T, while a negative MR behavior was recognized above 0.5 T. The slope of the positive MR decreased with increasing the temperature and was well fitted to the Brillouin function with S=5/2. The electrical and magneto-transport properties were very similar to those of Zn0.88Mn0.12O/ZnO heterostructures without doping Mg.