Controllable synthesis of Ru nanocrystallites on graphene substrate using polyol methods was reported herein. The zeta potential of graphene oxide dispersed in polyols is found to play a key role in tailoring the morphology and oxidation state of Ru nanocrystallites on graphene substrate. Negatively charged graphene oxide can coordinate with Ru3+ precursors via its carboxylic acid groups to form C-O-Ru complexes, leading to the epitaxial growth of hexagonally shaped cationic ruthenium nanocrystallites of 3-5 nm, which shows high catalytic activities for ammonia synthesis as expected; whereas positively charged graphene oxides can only interact with Ru3+ precursors via Van der Waals forces, resulting in a significant agglomeration of metallic ruthenium nanoparticles and thus a low catalytic performance. The morphology and oxidation state of ruthenium nanocrystallites supported on graphene substrate, thus the catalytic activity for ammonia synthesis, can be tailored by tuning the zeta potential of graphene oxide dispersed in polyols.