Various solutions for data transmission over multiple uplink carrier with respect to user equipment (UE) in mobile communications are described. A UE may establish a connection over a downlink component carrier and a first uplink component carrier with a network apparatus. The UE may further establish a connection over a second uplink component carrier with the network apparatus. The UE may transmit uplink data to the network apparatus via at least one of the first uplink component carrier and the second uplink component carrier. The UE may also assign the first uplink component carrier as a primary carrier and assigning the second uplink component carrier as a supplementary carrier. The UE may further switch the primary carrier from the first uplink component carrier to the second uplink component carrier.