The purpose of the study was to examine science teachers' knowledge structures on technology, who participated in a TPACK-based Professional Development (PD) program. The PD program was executed in the summer of 2015-2016 academic year with 24 science teachers. Data was collected with the Word Association Test (WAT). A holistic case study approach was followed throughout the study. The stimulus words used in WAT can be stated as; "technology," "information and communication technologies," "computer," "instructional materials," "Web 2.0 tools," and "Technological Pedagogical Content Knowledge (TPACK)." Cut-off points were identified by the frequency tables of response words towards stimulus words. The cut-off points helped create the concept networks on technology. At the end of the PD program, progress in science teachers' knowledge structures toward technology was observed. The results showed that the 21st century technologies included in the PD program such as Web 2.0 tools, Algodoo, animoto, probeware, and student response systems were found to be prominent in teachers' responses.