Background Training both pre - and in-service teachers who are capable of planning and enacting engineering design-based science instruction are critical for integrating engineering into K-12 science curricula. Purpose This study investigates pre-service teachers' efforts to integrate engineering into their lesson plans. Sample 13 pre-service chemistry teachers, enrolled in a 13-week engineering-infused integrated STEM course, were the participants of the study. Design and Method Considering the purpose of the study, qualitative design was deemed to be the best fit including qualitative data collected through lesson plans and reflection papers. To assist pre-service teachers' learning, 13-week engineering-infused integrated STEM was started with the information about the engineering profession and how engineers work. Additionally, pre-service teachers engaged in six integrated STEM activities including engineering design cycle. After each activity, the participants were asked to write a reflection paper on what they learned about engineering, the engineering design process, and integrated STEM. Finally, the participants had three chances for analyzing strong and weak integrated STEM lesson plans and planning integrated STEM lesson planning experience with mentors. The pre-and post-course lesson plans (n=26) were analyzed using existing codes from the related literature, which is a deductive coding process. Later, an inductive approach was used to determine the categories of engineering integration into the lesson plans. Four categories (ranging from a complete engineering lesson to a science lesson with no engineering link) were formed. Reflection papers were used to support the findings. Result The results showed that none of the pre-plans included engineering. However, after participated in a 13-week integrated STEM course, all post-course lesson plans integrated engineering through the use of engineering design processes to some extent. Conclusion The training with different experiences seems promising for pre-service teachers to learn engineering integration into STEM lesson plans.