By means of the weakly guiding approximation, the mode spot sizes Wx and Wy of the fundamental mode along the semimajor (x-direction) and semiminor (y-direction) axes of the fiber core in elliptical core two-mode fiber are discussed. The variation of their ratio value Wx/Wy with the operation wavelength \lambda and the length ratio a/b between the semimajor axis and the semiminor axis of the fiber core is analyzed. Based on this analysis, the distribution figures of two-lobe interferential mode patterns are evaluated and simulated quantitatively for different phase difference changes between LPLP01 and LP11 modes. The two-lobe interferential mode patterns have the same profile and distribute symmetrically when the phase difference equals \pi. Their central distance S becomes larger when Wx/Wy augments gradually. Furthermore, the equation about the central distance S of the two-lobe interferential mode patterns is given when the operation wavelength varies between 0.65 and 1.31 \mum, which is important to applications shuch as sensors and coupling devices between different fibers.