Abstract: 45 elements, Al, As, Au, Ba, Br, Ca, Ce, Cr, Co, Cs, Dy, Er, Eu, Fe, Gd, Hf, Ho, K, La, Lu, Mg, Mn, Mo, Na, Nd, Ni, Pr, Rb, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Th, Ti, Tm, U, V, W.Yb, Zn and Zr, have been determined in IAEA Lake sediment RM, SL-3 by reactor neutron activation analysis. Existing reactor NAA techniques of all kinds and newly developed methodology have been used in an effort to tap the potential of NAA as far as possible in terms of the number of determinable elements and the accuracy in the determination of elemental concentrations. Among them, delayed neutron counting was used to determine U ,radiochemical separation was used to determine all 14 naturally occurring lanthanides, newly developed IK, method was used in the corrections for interferences from fission reactions and γ-ray spectra.Concentrations for 26 elements have been certified by IAEA. Among them, our results for 25 elements are in good agreement with certified values within 95% confident limits. The others are also in good agreement with the means of accepted lab. averages summarized by IAEA.