Of all multiferroic materials, BiFeO3 stands out particularly with its both magnetic and ferroelectric transition temperatures above room temperatures, more interestingly having incommensurate magnetic structure with an extremely long period of 630 A. Using 10 co-aligned single crystals of BiFeO3, we have recently succeeded in measuring the full spin wave of BiFeO3 at MERLIN with the incident energy of 250 meV (see Experimental Report RB1120331). These data are now analyzed by us using a Heisenberg Hamiltonian with two exchange integrals. Unfortunately, however, our MERLIN data suffer from large backgrounds of the elastic peaks and do not give sufficiently good information on the low energy spin wave, where we expect to see the effects of a Dzyaloshinskii-Moriya interaction. In order to solve this problem, here we propose to measure BiFeO3 by rotating the sample at LET.