Liquid crystalline (LC) phase transition and gel-sol transition in the solutions of microcrystalline cellulose (MCC) and ionic liquid (1-ethyl-3-methylimidazolium acetate, EMIMAc) have been investigated through a combination of polarized optical microscope (POM) observation and rheological measurements. Molecular LC phase forms at the 10 wt % cellulose concentration, as observed by POM, whereas the critical gel point is 12.5 wt % by rheological measurements according to the Winter and Chambon theory, for which the loss tangent, tan delta, shows frequency independence. Dramatic decreases of G' and G '' in the phase transition temperature range during temperature sweep are observed due to disassembling of the LC domain junctions. The phase diagram describing the LC phase and gel-sol transitions is obtained and the associated mechanisms are elucidated. A significant feature shown in the phase diagram is the presence of a narrow lyotropic LC solution region, which potentially has a great importance for the cellulose fiber wet spinning.