Smartphone has been one of most important technologies in recent years. However, smartphone only has a limited computing capability and a relatively small storage, because a smartphone must conserve power in order to achieve longer operation time. Due to its limited storage space it is impossible to store all the personal data into a smartphone. In addition, the CPU of a smartphone runs at relatively low clock rate so we cannot run resource demanding application because of the limited computing power. We designed a file system for smartphone called NexusFS to overcome the inconveniences while using a smartphone. NexusFS can provide smartphone user Tera bytes of cloud virtual storage, which users can store all their personal data. NexusFS provides a cache mechanism on the smartphone. The NexusFS caches the user recently used files for user which gives good user experiences in accessing file in the cloud under unstable network environment such as 3G network, and supports disconnected operations on the mobile phone. The smartphone user can edit files on the smartphone transparently even without network accessing. NexusFS also provides version controlling mechanism which enable users to cooperate on the shared files on the cloud. With NexusFS, the smartphone users can access the files on the cloud more efficiently, and the smartphone application developers can use the cloud storage resource as the POSIX [1] standard local file system without using special API, which enables general applications on the smartphone to use the cloud storage resource without modifying. Smartphone application developers can concentrate on development of the application without concerning how to connect to a cloud storage service.