Premastication of natural rubber (NR) is a common process to ease the incorporation of fillers and chemicals. This research studied the changes in molecular structure, molecular weight (MW), and linear viscoelastic (LVE) properties of the standard Thai NR block grade STR5L, which is the low contamination grade, after mastication for 20 min at 40, 70, and 100 rpm, respectively. The MWs of unmasticated and masticated NRs were quantified by size exclusion chromatography. The "dual constraint" model, which is known to provide good predictions of the LVE properties from the MW and molecular weight distribution (MWD) of linear polymers, was used to correlate the MWs of NRs to their LVE properties. It was found that the dual constraint model could predict the dynamic moduli of NRs masticated at high speeds (70 and 100 rpm), indicating that the rheological behavior of the masticated NRs at 70 and 100 rpm conformed to linear polymers. On the other hand, the predictions for the unmasticated NR and the NR masticated at a lower speed of 40 rpm (NR40 rpm) deviated from the experimental values, especially in the terminal zone. This deviation might be due to the presence of physical gel and microgel in NR, which might still exist at mastication temperatures below 100 A degrees C.