The objective of this research and development was to develop an effective blended instructional model via weblog to enhance English summary writing ability of Thai undergraduate students. A sample group in the English program of Nakhon Ratchasima Rajabhat University was studied in academic year 2010-2013. The research instruments were an effective semi-structured interview form, the learning and instructional record format, the test of English summary writing ability, and the English summary writing criterion. The data were analyzed by percentage, means, standard deviation, and the t-test. It was found that the blended instructional model via weblog comprised three stages of blended activities: extracting information via face to face instruction, summarizing via weblog, and publishing on weblog. It was named "S2A Model." The model was effective in enhancing the university students' English summary writing ability as the post-test scores of the students met the set criterion of the study.