The structures and nonlinear optical (NLO) properties of the charge transfer anion-radical salts without excess electron, M(+)TCNQ (M = Li, Na, K) are studied for the first time. These anion-radical salts exhibit large first (beta(0)) and second (gamma(0)) hyperpolarizabilities, which are related to the ligand-to-metal charge transfer (LMCT) transitions. The largest beta(0) and gamma(0) values for K(+)TCNQ are 2.8 x 10(4) and 2.0 x 10(6) a.u., respectively. In addition, the effect of alkali-metal doping and monotonous dependence on the alkali atomic number of both beta(0) and gamma(0) are observed. These results may be beneficial to design high-performance NLO materials. (C) 2008 Elsevier B.V. All rights reserved.