The invention provides a method for improving a buffer status triggering mechanism for a mobile device of a wireless communication system. The method comprises: receiving an indication indicating that uplink data becomes data available for transmission in an upper layer of the communications layer, when data available for transmission other than the uplink data is included in a transmission buffer of the upper layer, determining whether a logical channel corresponding to any data of the uplink data has higher priority than logical channels corresponding to the data available for transmission in the transmission buffer, and when the logical channel corresponding to any data of the uplink data has higher priority than the logical channels corresponding to the data available for transmission in the transmission buffer, triggering a buffer status report.