This paper proposes BuOA (Business unit Oriented Architecture)

a novel architecture style for enterprise Web applications.Compared with the traditional layer architecture style

BuOA vertically decomposes a Web application into a group of BUs (Business Units)

each of which implements a complete and cohesive business function.To establish loosely coupled relationship between BUs

interactions between them are categorized into four patterns:observing

injecting

weaving and binding.We also provide development toolkits and runtime environment to support development and deployment of BuOA-based applications.Based on a concrete example

we show that BuOA is able to manage system complexity well on design

implementation and deployment stages in software lifecycle

with favorable support on parallel development and dynamic evolvement for enterprise Web applications.