In the present work, the crystal structure, transforming property, magnetocaloric and negative thermal expansion effects were investigated in polycrystalline Ni55.5Mn19.5Ga25 alloy, respectively. The structure measurement indicates that it possesses unmodulated martensitic structure (L1(0)) at room temperature. Here, the present alloy shows a first-order magneto-structural transformation from paramagnetic (PM) austenite to ferromagnetic (FM) martensite on cooling. Benefiting from this transformation, a considerable refrigerant capacity (RC) with a value of about 55 J/kg K was obtained around room temperature for the magnetic field change of 3 T. Moreover, the unique negative thermal expansion (NTE) effect was observed not only for the transforming range, but also for the pure martensitic state of the studied alloy. As distinct from a large NTE effect caused by magneto-structural transformation, such an effect at martensite is not accompanied by any thermal hysteresis, performing an excellently linear feature with a coefficient over 8 x 10(-6)/K within a broad temperature range (from 250 K to 50 K). (C) 2016 Elsevier B.V. All rights reserved.