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Recently Nakai et al. reported a theoretical H – T phase diagram of flux line lattice (FLL) structure in which successive transitions from a triangular, a square (\(\square_{\text{v}}\)), a triangular and another square (\(\square_{\text{g}}\)) occur with increasing a magnetic field. Here \(\square_{\text{v}}\) and \(\square_{\text{g}}\) indicate the FLL structures reflecting anisotropies in the Fermi velocity and the superconducting gap, respectively. In the case of YNi 2 B 2 C, \(\square_{\text{v}}\) and \(\square_{\text{g}}\) should rotate by 45°. The low field transition from triangular to \(\square_{\text{v}}\) is observed in RE Ni 2 B 2 C ( RE =Er, Tm, Lu, and Y). However, there is no experimental evidence for the appearance of \(\square_{\text{g}}\) phase so far. We studied the FLL structure of YNi 2 B 2 C in the higher field region by small-angle neutron scattering. Our results show that a large area of the H – T phase diagram is occupied by \(\square_{\text{v}}\) phase and there is no evidence fo...