Transport properties of two_dimensional electron gas (2DEG) are crucial to metamorphic high_electron_mobility transistors (MM_HEMT). We have investigated the variations of subband electron mobility and concentration versus temperature from Shubnikov_de Hass oscillations and variable temperature Hall measurements. The results indicate that the electrical performance is the best when the In content is 0.65 in the channel for MM_HEMT. When the In content exceeds 0.65, a large lattice mismatch will cause dislocations and result in the decrease of mobility and the fall of performance in materials and devices.