The article deals with the historical-theoretical analysis of the features of a promising constitutional process during the period of the Ukrainian People’s Republic (1917–1920). This is in particular about the experience of building a constitutional state, the relevance today evident in view of preparation of the Concept of amendments to the current Constitution of Ukraine. The attention is focused on the essence of the constitutional state, the necessity and importance of constitution of the natural law of the sovereign on the implementation of the constituent power with clearly defined conditions for access to power, which is the basic and primary condition for the establishment of the constitutional system.