A systematic investigation of microstructure and crystallography of the multiply twinned structure at the nanometer scale in a Ni 53 Mn 25 Ga 22 ferromagnetic shape memory alloy (FSMA) was performed. Nanoscale internal twins inside the micrometer-scale martensitic lamellae were observed. In each lamella, two nanotwin variants with a compound twinning relationship exist, and the twinning elements are K1 ={1 1 2}; K2 = {1 1 2}; n1 = {1 1 1}; n2 = {1 1 1}; P = {1 1 0} and s = 0:379. Two types of lamellar interfaces, i.e., interpenetrated interlamellar interface and stepped intralamellar interface, were revealed. The orientation relationships between the nanotwins connected by these two types of interfaces were unambiguously determined, and it was found that the respective orientation relationship between the neighboring nanotwins depends strongly on the adjacency condition at the interfaces. The possible formation mechanisms of the interpenetrated interlamellar interface and stepped intralamellar interface are discussed. These results are useful for property optimization by microstructure control in the FSMAs.