We classified students' mental models on conductors and dielectrics into four levels that vary progressively from naive views to scientifically acceptable views, namely non-model, non-scientific model, flawed-scientific model, and scientific model. A total of 137 Chinese students from an introductory physics class completed a questionnaire that targets learners' conceptual understanding of conductors and dielectrics. For the questions on conductors, about 20% of the students were at the non-model level, and almost 15% of the students constructed a non-scientific model and believed that both electrons and protons could move in a conductor. Those with a flawed-scientific model (35%) were unsure about the motion of the particles. Only 30% of the students had a scientific model. For the dielectrics questions, over 30% of the students were in the non-model level; about 55% of the students had a non-scientific model and thought that charges could not move in a dielectric. The remaining 10% of the students used a flawed-scientific model.