By electrostatic repulsion and steric hindrance, polycarboxylate-based superplasticizer (SP) can decrease flocculation of fine powder and enhance the flowability of mortar. As a result, less water is required at a given flowability and thus adding SP could enhance the overall behavior of mortar. Nonetheless, SP also introduces unfavorable shear thickening in mortar due to the clustering of mobile and adsorbed polymers that forms dead zone of flow and increase flow impedance, which decreases the uniformity of mixing and pumpable distance of concrete, as well as creates difficulty in in-situ handling. It is now clear that producing high-performance mortar required adding SP, which would unevitably cause shear thickening. This paper employs a coaxial rheometer to investigate the dilatancy of cementitious mortar, with or without the partial substitution of cement by inert limestone powder. Results show that the shear thickening of mortar increases initially due to the introduction of SP but decreases after reaching the critical SP dosage. It is because adequate SP disperses effectively the cement particles for better hydration, which increases the formation of ettringite hence absorption of mobile SP polymers. More importantly, adding inert limestone powder and at the same time, situable amount of SP could reduce the shear thickening due to the improved wet packing density (WPD).