Nanoparticles have emerged as innovative in several fields, including agriculture, because of their unique physical and chemical properties. Nanoparticles measuring less than 100 nm enter plant cell membranes easily yet produce reactive oxygen species that cause cellular damage through oxidative stress. The unique physicochemical characteristics of nanoparticles affect the complex process of NP-plant interactions. This chapter discusses plant nanoparticle toxicity’s main factors: size, shape, surface charge, coating, aggregation state, hydrophobicity, porosity, and ligand binding. The effects of these variables on oxidative stress, development, and metabolism are described in more detail below. This chapter clarifies the complex interplay between NP properties and plant responses, providing a framework for the development of nanotechnologies that balance agricultural benefits with environmental safety.