The growth morphology and crystallinity of the electroless NiP deposited on Si substrate pretreated by SnCl2/HCl and PdCl2/ HCl solutions is studied using transmission electron microscope (TEM), field emission scanning electron microscope (FESEM), and energy dispersive X-ray spectrum (EDS). Combining cross-sectional and surface observations show that the reaction mechanism of electroless NiP on Si is an electrochemical mechanism and the deposits are composed of a columnar structure grown along the vertical direction of the substrate surface. The phosphorus content does not have influence on the growth morphology but affects the crystallinity of the deposits. The crystallinity of the deposits is directly observed by the lattice image using high-resolution transmission electron microscope (HRTEM). The as-deposited NiP with 10.7 at.% P possesses good crystallinity and consists of Ni nanocrystal about 4-8 nm distributed randomly in the deposits. The size of nanocrystal in the deposits with 15.2 at.% P is about 2-5 nm. The deposits with 20.3 at.% P have a smaller order range and the size of nanocrystal is under 1.5 nm. The grain size decreases as the phosphorus content increases. (C) 2004 Elsevier B.V. All rights reserved.