Microplastics and nanoplastics are emerging pollutants of concern in the aquatic environment that are causing increasing global environmental and human health problems. Although there has been extensive research on microplastics and nanoplastics, little has been said about the differences in their behavior in the aquatic environment, and many studies have considered them as the same class of hazardous materials; but in fact, microplastics and nanoplastics should be considered as two different types of environmentally hazardous materials. In this review, we propose that microplastics and nanoplastics behave in the aquatic environment in a size-dependent manner and should be distinguished. And we systematically analyzed the differences in the behavior of microplastics and nanoplastics in the aquatic environment in terms of five aspects: 1) distribution behavior; 2) adsorption behavior; 3) reaction with natural colloids; 4) aging and leaching behavior; 5) interaction with organisms. This paper has been written to draw academic attention to the different behaviors of microplastics and nanoplastics in the aquatic environment in order to distinguish between their effects on humans and the environment.