Inhibition effect of Hexa propylene glycol cyclotri phosphazene (HPGCP) was studied on carbon steel cor rosion in 1 M HCl has been investigated in a temperature rangin g from 298 to 328 K using weight loss measurements and electrochemical polarisation. The inhibition effici ency of HPGCP decreases with the rise of temperatur e. The values of adsorption enthalpy a H ∆ and adsorption entropy a S ∆ were discussed. The thermodynamic parameters of adsorption reveal that there is a strong interactio n between inhibitor and carbon steel. The highest i nhibition efficiency was observed 10 -5 M of HPGCP concentration at different temperature