In the study presented in this paper, we examined the possibility to scaffold mathematical modelling with strategies. The strategies were prompted using an instrument called "solution plan" as a scaffold. The effects of this step by step instrument on mathematical modelling competency and on self-reported strategies were tested using nineth grade students (N = 91) at a German middle track school (Realschule) in a quasi-experimental design. Six classes were randomly assigned to the experimental group, in which students used the solution plan, or to the control group. The quantitative data analysis using ANOVAs reveals that (1) in the posttest the experimental group students reported more frequently about planning, rehearsal, elaboration and organizing strategies while solving modelling problems than the control group; (2) the "solution plan" as a scaffold supports the development of students' modelling competency, including its sub-competencies. The students who used the solution plan outperformed the other students in solving modelling problems concerning the topic "Pythagorean theorem".