Disclosed is an optical network unit (ONU) capable of reporting current Dynamic Bandwidth Report upstream (DBRu) information to an optical line terminal (OLT) according to the amount variation of to-be-transmitted upstream data in a buffer. The ONU includes: the buffer temporarily storing the to-be-transmitted upstream data; a register circuit recording previous data amount information related to the previous data amount of the buffer at a previous time point earlier than a current time point; a DBRu information generating circuit generating the current DBRu information according to an amount difference and a current data amount of the buffer at the current time point, wherein the amount difference is dependent on the difference between the previous data amount information and current data amount information that is dependent on the current data amount; and a transmitting circuit transmitting the current DBRu information to the OLT.