Currently, the research of IEEE 802.15.4 rarely relates to the network initialization. However the network establishment and the device association are also the important part of IEEE 802.15.4. This paper analyzes network initialization flows in detail and develops an improved association scheme named Simple Association Process (SAP). It is introduced to reduce redundant primitives, avoid collisions and decrease association delay. Comparing SAP with the original IEEE 802.15.4 protocol, the number of association service primitives in SAP is 31.3% less than in the original protocol, and the simulation results show that the association time decreases 64.5%. SAP will get better performance as the number of devices increases and the interval time of association request decreases. It can be widely used in home networks and sensor network applications.