The martensitic transformations of seven Ni-Mn-Ga alloys were investigated by the measurements of resistivity, susceptibility and dilation. Two step martensitic transformations occur in polycrystalline Ni-18.4wt%Mn-29.5wt%Ga, Ni-18.2wt%Mn-28.8wt%Ga and Ni-19.5wt%Mn-27.2wt%Ga alloys. The first martensitic transformation is thermoelastic, which exhibits a steep increasing in resistivity, decreasing in susceptibility and contracting in sample length along the direction of ingot. The second martensitic transformation exhibits a larger thermal hysteresis, and opposite changes in resistivity, susceptibility and thermal expansion compared with the first transformation. No martensitic transformation exists in the alloy with higher Ga content than that of the above-mentioned three alloys. The alloys with lower Ga content show a single martensitic transformation and higher transformation temperatures.