Order-disorder phase transition in Ni_{2+x}Mn_{1-x}Ga Heusler alloys has been studied. It was found that the L2_1 \to B2^{\prime} phase transition in Ni2+xMn1-xGa (x = 0.16 - 0.20) Heusler alloys is of second order and the temperature of this transition decreases with Ni excess.