Dynamics of the formation of psycho-physical scales in 7-16 years old school-children is studied by means of psycho-physical (subjective) scaling. It is shown that formation of a scale goes through three stages corresponding to junior, middle and senior school age. The first stage is characterized by the formation of interval scales, the second - by a transition to relational scales, the third - by final cycle of transition to mentioned scales. The character of changes in psycho-physical scaling shows that more advanced level (or less advanced one) is achieved after attaining a maximum. These parameters, changing with age, clearly mirror specificity of child-ren's development, so the given method can be used in developmental psychology and psycho-physiology.