For studying the interaction between cellulose ethers and cement pastes at early ages, FTIR spectra and thermal analysis were performed to investigate the influence of HEMC(hydroxyethylmethyl cellulose) on the formation process of major hydration products in cement pastes at first 24h . The results indicate that the HEMC prolongs the formation of ettringite, CSH gels and CH(calcium hydroxide), and retards the transformation of absorbed water to crystal water in hydration products. Moreover, the retardation capacity of HEMC is different for different hydration products, in which the strongest is for CH and weaker is for ettringite and CSH gels. No new products are detected in cement pastes modified by HEMC compared to neat cement pastes.