Providing scientific explanations for natural phenomena is a fundamental aim of science; therefore, scientific explanation has been selected as one of the key practices in science education policy documents around the world. To further elaborate on existing educational frameworks of scientific explanation in K-12, we propose a learning progression of scientific explanation based on the Phenomenon-Theory-Data-Reasoning (PTDR) framework. To examine the validation of the PTDR framework for scientific explanation and the learning progression based on it, we developed the "Scientific Explanation Progression Assessment" to investigate students' performance (Grades 8-12 students, N = 4,554) on scientific explanations. A Rasch analysis and other statistical tests together indicated the validity of the PTDR framework for scientific explanation and the learning progression derived from this framework. We end with a discussion of the implications for future design of instruction and assessment.