When an external electric field is applied to poly(ethylene glycol) (PEG) particles suspended in silicon oil, an electrorheological (ER) effect is observed by which the viscoelastic properties of the suspension change. We previously found that PEG of low molecular weight enhanced the ER effect, which suggests that hydroxyl end groups rather than ether groups of main chains contribute to the effect. In this study, we synthesized PEG with acetate and carboxyl end groups and stearated PEG (Emanon) and studied the ER effect of these particle suspensions. Acetate and stearate resulted in a weaker ER effect than that of normal PEG (with hydroxyl end groups), whereas carboxylated PEG produced a stronger ER effect. The chain structure of carboxylated PEG particles in an external electric field was thicker than that of normal PEG. From the calculation of dipole moment of alcohol, esters and carboxylic acids, the difference in ER effect between PEG with various end groups was not caused by the strength of the dipole. When hydroxyl groups are esterified, hydrogen bonding among these groups disappears. However, because the carboxyl group has two hydrogen bonding sites, it is thought that the enhanced ER effect of the carboxylated PEG suspension was due to hydrogen bonding among particles.