This research aims to describe the characteristics and implementation as well asto describe the response of the teachers and the students towards the implementation of the assessment instrument on the students? critical thinking ability with the learning model of Creative Problem Solving. This research is a developmental research. The results of this research are the characteristics of the assessment instrument which consist of two parts, namely an instrument to measure the psychomotor and cognitive domain, specially designed to measure thestudents? critical thinking ability using the learning model of Creative Problem Solving(CPS). The use of assessment instrumentsin learning is effective to improve the students? critical thinking ability. The response of the teachers and the students about the assessment instrument that has been developed is good and worthy to be used in the learning.