Two new kinetic hydrate inhibitors (KHIs) named PVP-B and PVP-BP were synthesized successfully and compared with commercial KHIs such as PVP and Inhibex 55W. PVP-B and PVP-BP are both the ramifications of PVP. They were obtained by introducing a t-butyl group and both t-butyl and phenyl groups into PVP molecules, respectively. The hydrate inhibition performances of KHIs were assessed in a sapphire cell through two kinds of onset times of hydrate formation: TVO, the time when hydrate crystals are initially observed by naked eyes, and TPD, the time when rapid and continuous dropping of system pressure begins. TVO Was used to evaluate an KHI's inhibition performance to hydrate nucleation. The length of time period from TVO to TPD was used to evaluate a KHI's inhibition performance to hydrate growth. The results demonstrate that both PVP-B and PVP-BP are superior to PVP and Inhibex 55W; PVP-BP has better inhibition performance than PVP-B because of the stronger steric hindrance effects of the phenyl group in PVP-BP molecules than those of the t-butyl group in PVP-B molecules. Additionally, we found TVO does not always increase with the increasing dosage of KHI while TPD does. The most suitable dosage of PVP-BP was determined to 0.5 wt % or so, at which the nucleation of hydrate is inhibited most perfectly. Finally, we demonstrated glycol could be used as synergist to improve the performance of PVP-BP remarkably. The most suitable dosage of glycol was determined to be 0.9 wt % or so. Our work presents not only two new KHIs but also important insights into the inhibition mechanism of KHIs.