STEM education curriculum is an effort mainly targeted for stakeholders to increase students' interest in STEM as well as to meet future workforce demand. Teachers’ instructional preparations are an important part to the overall effectiveness of the teaching and learning process. This study focuses on the development and validation process of STEM Teachers' Instructional Preparedness Instrument (STEMTIP). The study comprises ten steps ranging from development to validation phase. Social constructivist theory, 5E instructional model and STEM teaching and learning approach model were used as the basis for the development of this instrument. There were 51 items initially generated using inductive and deductive approaches which include face validity, content validity, construct validity and criterion validity as were confirmed by the experts’, and statistical results of Rasch model and regression analysis. The reliability analysis also demonstrated acceptable consistency for the instrument. After going through the validity process, a total of 41 items remained as final items for this instrument. Full details of reliability analysis, validation along with implication for practice are discussed.