This paper discusses two new polyolefin elastomers, i.e., poly(ethylene-ter-prepylene-ter-p-methylstyrene) (EP-p-MS) and poly(ethylene-ter-1-octene-ter-p-methylstyrene) (EO-p-MS), containing "reactive" p-methylstyrene (p-MS) groups. Both terpolymers, with a broad range of terpolymer composition, high molecular weight, narrow molecular weight, and composition distributions, have been prepared by using [C5Me4((SiMe2NBu)-Bu-t)]TiCl2 metallocene catalyst with constrained ligand geometry. Differential scanning calorimetry (DSC) results show no detectable melting point (T-m) in both terpolymers containing less than 70 mol % ethylene units. The sharp glass transition temperature (T-g) with flat base line in each DSC curve indicates homogeneous terpolymer microstructures. In general, the T-g of EP-p-MS is very sensitive to the terpolymer composition (ethylene/propylene ratio and p-MS content;). In the compositions with the ideal ethylene/propylene ratio similar to 55/45, T-g is almost proportional to the content of p-MS. Only with the concentration of p-MS <2 mol % does the EP-p-MS show a low T-g < -45 degrees C. On the other hand, the EO-p-MS system exhibits a low T-g < -50 degrees C in a wide range of copolymer compositions, even ones with relatively high p-MS contents (>7 mol %).