The object of this study are the works of Finnish composer Usko Meriläinen (b. 1930), from his early neo classical compositions of the late fifties to his mature post serialistic style of the eighties. The study representsscore-centered analysis in which the main research materials are the scores of Meriläinen's compositions. The main hypothesis for the analyses is that Meriläinen constructs his compositions by means of gradual ransformation and metamorphosis of the basic material which is introduced in the very beginning of each composition. In Meriläinen's works this basic materialcovers elements of pitch organisation, harmony, rhythm and timbre and so the analysis must cover all parameters of music instead of only one which usually is pitch organisation and / or harmony. The main object of theanalyses are the structures as well as the equality of musical parametres in Meriläinen's compositions. The study defines how the small constituents of the compositions are made up (micro analysis), how these constituents unite in larger sections (middle analysis), and finally how these sections together form a harmonious whole - i.e. a composition (macro analysis). In other words this study seeks an answer to a question how Meriläinen compose. The analyses shows that the works of Meriläinen are very coherent and of high standand. Stylistical changes are not abrupt but gradualshiftings from one stylistic conception to an other. He uses no pre-calculated schemes but instead relies on his intuition. The analyses shows that each composition by Meriläinen is structurally unique and no identical structures exist among his compositions. The transformation and metamorphosis technique of musical characters becomes a norm on his Second Piano Sonata (1966) at the latest but as early as in his Concerto forOrchestra (1956) there are signs of this technique. The tone language of Meriläinen's works is modern (post serial) but the structures of his compositions are indisputably tied up with tradition.