Abstract: A wireless device and a random access method thereof for a mobile communication system are provided. The mobile communication system defines a plurality of preambles and a plurality of preamble subsets. Each preamble subset includes a part of the preambles and the union of the preamble subsets is all of the preambles. The preamble subsets have a set inclusion relationship therebetween, i.e., the larger preamble subset includes the smaller preamble subset. The wireless devices have priority levels and the wireless devices with different priority levels can use different groups of preamble subsets. When performing the random access procedure, the wireless device selects a preamble from the smallest preamble subset corresponding to its priority level. Afterwards, when the request for random access fails, the wireless device sequentially selects a preamble from the larger preamble subset.