Crystallographic structures and orientations of nanoscale iron particles encapsulated in carbon nanotubes (CNTs) were studied with or without heat treatments at various temperatures. It was found from this study that gamma-Fe with a face-centered-cubic crystal structure (known to be stable in the temperature range of 912-1394 degrees C for bulk iron) retains its stability in a large quantity at room temperature inside the cavities of CNTs. The relative number of gamma-Fe particles was found to increase with temperature, especially with heat treatment at 1400 degrees C or higher. It was also found that iron particles in CNTs are oriented in specific crystallographic directions of their crystal structures along the axis of the nanotubes. (c) 2004 Elsevier B.V. All rights reserved.