The present study examines the influence of chemical composition and structure of guar gum derivatives on water retention capacity (WR) and rheological behavior of fresh state cement-based mortars. The investigation was also completed by adsorption isotherms. For this, original guar gum, three HydroxyPropyl Guars (HPG) and two hydrophobically modified HPGs were selected. The effect of the molar substitution (MS_HP) and of hydrophobic substitution (DS_AC) was investigated. The results highlight that chemical composition of HPG_s has a remarkable effect on fresh state properties of mortars. The original guar gum does not impact on neither WR nor rheological behavior. Increasing MS_HP leads to an improvement of the WR and the stability of mortars while the hydrophobic units further enhance WR and lead to a decrease in the yield stress and an increase in the resistance to the flow of admixed mortars.