The study aimed to determine the effect of a brain-based environmental program for preschool children aged 5-6 years on their sustainable environmental behaviors. For this purpose, an experimental study was designed for a period of 8 weeks. The participants of the study were composed of 20 children from middle socio-economic level families who attend two branches of a kindergarten in the province of Kütahya. Out of those 20 children, 10 were in the experimental group and 10 in the control group. The program applied to the experimental group was prepared by the researchers considering the principles of the brain-based learning approach. The 16 activities prepared for pre-school children were finalized by taking expert opinion from 5 experts in the field of pre-school education. The classrooms where the activities were implemented were arranged according to brain-based learning (such as heat, light, visual materials), fruits and drinking water were kept at the activity tables in order to meet the glucose and water needs of the brain, which are important in brain-based learning and that the brain needs for learning. Relaxation exercises were carried out and classical music was provided during all activities. Activities were held outside whenever the weather conditions are suitable. In the control group, the activities were prepared by the researchers according to the achievements in the 2013 Preschool Program of the Ministry of Education (MoNE). In the pretest and posttests, the "Environmentally Sustainable Behavior Assessment Scale for 60-72 Months-Old Children" developed by Özkan, Tugluk and Yigitalp (2019) was used. The scale consists of 20 items, consciousness and awareness, and 2 sub-dimensions. The scale is a 5-point Likert-type scale and is filled by the child's teacher for each child. Cronbach Alpha value of the scale; It is .92 for the awareness sub-dimension, .94 for the consciousness sub-dimension, and .92 for the whole scale (Özkan, Tugluk & Yigitalp, 2019). The results revealed that there was a significant difference between the pretest and posttest scores of the experimental group, and the posttest scores of the experimental and control groups.