A radio terminal according to one embodiment comprises a processor. The processor is configured to: forward a data unit from an RLC layer to a MAC layer prior to receiving an uplink grant from a base station; receive the uplink grant for determining a predetermined transport block size from the base station; generate a transport block by performing concatenation or segmentation on the data unit according to the predetermined transport block size in the MAC layer; and transmit the generated transport block to the base station via a PHY layer. The processor is configured to receive TBS information indicating a minimum guaranteed value of a transport block size from the base station. The predetermined transport block size is a value equal to or larger than the minimum guaranteed value.