Pupils who cannot perceive the environment as a complex, interconnected system tend to be less aware of environmental issues, a comprehension significantly influenced by the medium of instruction. Learning science in a non-native language can increase cognitive load, leading to difficulties in understanding concepts due to language barriers. This study compares mother tongue-based (MTB) and non-mother tongue-based (nMTB) pupils’ mental models of the environment by level of mental model, types of mental model and their pro-environmental behavior. A descriptive survey research design was employed with a sample of 60 Year Five MTB and nMTB pupils from two schools in an urban area in Kuala Lumpur. Pupils’ mental models were assessed using the Draw-an-Environment Test (DAET), and pro-environmental behaviors were measured through a questionnaire. Descriptive statistics and content analysis were used to analyze the data. The findings revealed that the environmental mental models of MTB pupils are slightly better than those of nMTB pupils and reveals distinct differences in the types of mental models. Additionally, the study found that the level of pro-environmental behavior among MTB pupil is higher than nMTB pupils. Overall the study indicates that MTB pupils are associated with slightly better mental models and pro-environmental behavior among pupils compared nMTB.