With potassium hydroxide (KOH) as catalyst, hydroxypropyl guar gum (HPG) was successfully prepared under homogenous conditions in an ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl). The value of degree of molar substitution (MS) of HPG, which was determined by means of (1)H NMR, was easily controlled by varying the mass ratio of propylene oxide (PO) to guar gum (GG). The distribution of hydroxypropyl moieties among the seven hydroxy groups in HPGs was investigated by NMR and the results indicated that the homogeneous reaction preferentially substituted at the C3−OH of the mannosyl residues in guar gum which was quite different from the heterogeneous reaction where substitution mostly occurred at C6−OH. The average number of hydroxypropyl units per OH substitution of HPGs with MS value of 0.20, 0.37 and 0.60 was 1.82, 1.42 and 1.76, respectively.