A network access method of Wireless Local Area Network (WLAN) terminals and a network system thereof which are designed to support network association between a terminal and an Access Point (AP) in an IEEE 802.11 WLAN environment includes: scanning at least one AP in the network and determining whether or not the AP uses a Wired Equivalent Privacy (WEP) Key; and converting to an Open System authentication mode or a Shared Key authentication mode according to the determination. Accordingly, the terminal can scan for the existence of a WEP Key from at least one AP and convert into a network-accessible Open System authentication mode and a Shared Key authentication mode so as to execute an association.