The aim of this article is to present the theoretical motivational background regarding metacognitive self, which is being aware of how biases and psychological rules (like classic conditional) influence one's own behavior. Based on this framework, we used a Polish nationwide representative sample to create a reliable tool (the first study: n = 1200, the next study n = 600, Partner in Business Strategy Company as an external contractor, who served as data collector). Until now, the MCSQ-40 questionnaire has been used. After modification -changing the continuous scale into a 6-point scale -and a survey of a representative sample of the Poles, a single-factor structure of metacognitive self was developed, and 21 items out of 40 previously used were selected. This resulted in developing a new tool: MCSQ-21. We assessed the congruent and concurrent validity of this instrument.