This study aimed at investigating the effectiveness of a suggested professional development program in enhancing nano-‎literacy for a voluntary sample of secondary school female science teachers in KSA. A quasi-experimental approach based on a one-group pretest-posttest design was used. The population included all female science teachers at secondary schools ‎in KSA. From these, a sample of (33) science teachers were selected voluntarily. Data was collected by the Cognitive Aspect of Nano-Literacy Test (Al-Atiyat , 2016). An online program focusing on six domains of the cognitive aspect of nano-‎literacy was developed. Results demonstrate the effectiveness of the program in developing nano-literacy as a total degree and sub-domains. The study recommended implementing the professional development program introduced in this study for developing the cognitive aspect of nano-literacy for pre- and in-service science teachers. It was also suggested to conduct a study investigating the effectiveness of a program that ‎integrates the other aspects of nano-literacy (such as the skill dimension, critical thinking, ‎decision-making, and educational applications) in developing science teachers' nano-literacy.‎