Grain boundaries (GBs) have been extensively explored due to their effect on the mechanical properties of materials, while their impact on chemical properties has received little attention. I will discuss two examples to demonstrate the impact of GBs on the chemical and catalytic properties of nanomaterials. In the first example, scanning tunneling microscopy (STM) was used to probe epitaxial graphene and its interaction with gas molecules. I found that the GBs in graphene on Ru substrate are extremely susceptible to chemical attack by water, which can cut the graphene along GBs, followed by the intercalation of water under the graphene. This process produced a lot of graphene flakes, and I further revealed the superlubric sliding and electronic screening effect of the flakes using STM. In the second, I explored the role of GBs in metal nanoparticles for electrocatalytic CO2 reduction. I prepared defect-rich metal nanoparticles using e-beam evaporation and quantified the average GB density by transmission electron microscopy. The catalytic activity for electrochemical reduction of CO2 and CO is directly correlated with GB density in metal nanoparticles. This is the first quantitative study of GB effect in electrocatalysis, and provides a new design principle for nanoparticle electrocatalysts.