Abstract: The present invention relates to an improved process of preparation of idelalisib using a solid form of intermediate of formula (2); characterized by XRPD pattern having 2? values 5.5°, 10.9°, 14.3°, 16.9° (±0.2) the process comprising steps a. Reacting of compound of formula (1); ?with a chlorinating agent to obtain a product; b. Reacting of the product with (S)-2-((tert-butoxycarbonyl)amino)butanoic acid and a base, wherein pH of the reaction mixture is set between 7.5 and 10 to obtain compound of formula (2); c. Isolating a solid form of compound of formula (2); d. Transforming the compound of formula (2) into idelalisib.