This study investigated the influence of Science, Technology, Engineering, and Mathematics (STEM) based phenomenon learning on students' proportional reasoning. The embedded mixed method design was utilized with 25 senior high school students in the East Java Province, Indonesia at the 2019/2020 school year. The data were collected via written tests (pretest and posttest) and interviews to find the effect of STEM-based phenomenon learning on students' proportional reasoning. Data were analyzed with the Wilcoxon signed-rank test and categorized into five levels of proportional reasoning. The result showed that STEM-based phenomenon learning led to no statistically significant increase in the students' proportional level. There was also a slight decrease in the percentage of students who categorized as level 4 (ration). In particular, the qualitative analysis documented that the students only focus on the proportion of one variable that changes and does not pay attention to changes in other variables. The students also indicated to provide an inverse proportion.