Polyethylene terephthalates (PETs) with well-defined chemical structures were prepared by molecular design, and the effect of the chemical structure on the physical properties of PET was investigated. Hydroxyl-group end-capped PETs with eta (inh) = 0.4-0.6 dL/g exhibited a viscosity behavior similar to Bingham fluids, although other PETs with similar molecular weights (MWs) showed Newtonian flow behavior. This theological feature was more noticeable for hydroxyl-group end-capped branched PETs. In addition, hydroxyl-group end-capped branched PETs became solidlike from 80 rad/s as the frequency was increased. On the other hand, hydroxyl end-capped linear PETs showed a noticeable viscoelastic transition peak around 20 rad/s. High MW linear and branched PETs with eta (inh) greater than or equal to 0.9 prepared by multistep synthesis showed non-Newtonian fluid behavior.