This study aims to see an increase in students' ability to understand concepts after applying the TTW (Think-Talk-Write) learning model assisted by Pictorial Riddle-based Student Worksheet. The research method used is quasi-experimental. There were two experimental classes, namely the modeling class and the implementation class. The sample of this study was 24 students of class XI Science 4 as modeling class and 23 students of class XI Science 3 as implementation class in Prambanan High School 1 Yogyakarta. Data collection techniques were carried out by Pretest and Post-Test. Data were analysed using the Wilcoxon test and descriptive analysis to determine the N-Gain score. The results showed that the TTW (Think-Talk-Write) learning model assisted by Pictorial Riddle-based Student Worksheet could improve students' conceptual understanding abilities. This is indicated by the value of Asymp Sig.(2-tailed) smaller than 0.05 where 0.046< 0.05 and the N-Gain score is 0.2061 in modeling class and Asymp Sig.(2-tailed) smaller than 0.05 where 0.003< 0.05 and the N-Gain score is 0.2775 in implementation class. The conclusion is this model has an effect on the students' ability to understand concepts with a low increase category.