The aim of this study is to design learning activities that can support students to develop strategies for the addition of number 1 to 20 in the first grade by involving students' spatial structuring ability. This study was conducted in Indonesia by involving 27 students. In this paper, one of three activities is discussed namely ten-box activity. This activity was aimed to introduce and develop ten-structure to be a students' strategy in addition of number 1 to 20. The method was design research by designing learning activities involving spatial structuring ability. PMRI underlined the context and activity. The result of the study indicates that ten-box activities can help students to develop ten-structure as a strategy in addition of number 1 to 20. As a recommendation, PMRI can be implemented as an approach of teaching and learning addition 1 to 20.