A network device measures upstream bandwidth of a communication network. The communication network includes a server and a plurality of communication nodes. The network device transmits a sequence of Internet control message protocol (ICMP) request datagrams corresponding to a sequence of time to live (TTL) increased from a first value to a second value to the server via the communication nodes. The network device designates the communication node that transmits an overtime datagram first as the best communication node, transmits a plurality of ICMP detection datagrams to the best communication node, receives response datagrams and records receiving time thereof, and calculates the upstream bandwidth according to size of the plurality of ICMP detection datagrams and the recorded receiving time of the response datagrams.