The 21st century is the era of technology, in which many countries implement various reforms and policies related to education in general and science education in particular. The integration of STEM with the arts including the fine arts, the liberal arts, and the physical arts is a remarkable feature. It turns STEM into STEAM and also becomes the slogan for education reform in many countries around the world. On the other side, with the development of the new general education curriculum in 2018 towards a competency approach, teachers are required to have innovations in order to follow several educational trends. STEAM teaching competency is one of the necessary competencies of teachers in this period. This article proposes a toolkit to self-assess STEAM teaching competency for high school teachers. Based on consultation with various experts, the survey was conducted and experimented on 249 teachers; by synthesizing Cronbach’s Alpha and Exploratory Factor Analysis, five groups of factors and 37 criteria were identified. Teachers can use this toolkit to self-assess, thereby adjusting, sharpening, and developing STEAM teaching competency. As a result, teachers can design interesting science and STEAM lessons for students and accomplish the goals of the new general education curriculum in 2018 towards a competency approach.