This paper presents a theoretical analysis on the lateral spread of the distribution for implanted ions in amorphous targets. First, it is shown that the solutions of the second order differential equations concerning moments of the ranges are necessary and sufficient to estimate the lateral distribution, if the Gaussian distribution is assumed. The calculated results of , <ΔRp> and the lateral spread are presented. Next, it is shown that the actual distribution function along the lateral direction is approximately expressed by the complementary error function at the window edge, in case when ions are implanted through the mask-window. The variation of the lateral spread with incident ion mass is also calculated. From these calculations, it is concluded that the lateral spread can not be neglected in the case of light ions such as B+ incident to Si.