The otolith of fish is an important source of ecological information. It is a popular and successful tool in discriminating between and within fish stocks to study the morphologic characteristics of the otolith. A total of 222 pairs of sagittal otolith were collected randomly from Nov. 2009 through Jan. 2010 in waters near Marshall Islands. The morphologic characteristics of them were observed and measured. There were three channels on them which divide the sagittal otolith four areas (dorsum, wing, rostrum, and lateral). There was no significant difference between the morphological parameters of the left sagitta and the right sagitta (P=0.999). The result of principal component analysis on 12 morphologic parameters showed the dorsal length (DL), wing front length (WFL), wing length (WL) and maximum statolith thickness (MST) can be used to represent the size features of the sagittal otolith; the dorsal angle (DA), and wing angle (WA) might reflect its angle features; these six parameters can be used to indicate the morphologic characteristics of the otolith. The optimum relationship between DL and MST was described by exponential function, the optimum relationship between fork length (FL) and DL was described by quadratic multinomial, while the optimum relationship between FL and weight of sagittal otolith (OW) was the logistic function. The sagittal otolith was growing and the core of sagittal otolith was shifting to the dorsum, rostrum, and lateral areas while the fork length was increasing. There was fluctuation for DA when the length of DL was between 500 μm and 700 μm by One way random block design. The DL was about 600 μm and the forklength was about 80-130 cm while bigeye tuna was mature.