Two kinds of expanded core fibers (ECF) are fabricated, one is single-mode fiber cascaded gradual index (GI) fiber and the other is thermally diffused expanded core fiber (TEC). The testing shows that ECF can reduce loss in fiber-to-fiber coupling and increase the misalignment tolerance greatly, so that it make the alignment and adjustment easier than normal single mode fiber(SMF). The transmission characteristics of the two kinds of ECF are studied and the probable use in optical devices such as connector, isolator and optical filter working as collimator are analyzed.