This paper analyzes the influences on measured MFD （Mode Field Diameter） which are caused by three typical coupling states between the two fiber endfaces in measuring MFD of single mode fibers with transverse offset method. The paper also describes the mechanism and the size of the influences respectively. Based on the theoretical analyses and the measured results

the relations between the measuring errors of MFD and the deteriorations of transmission power due to each coupling state are clarified. Finally

the predominant conditions in measuring MFD are recommended.