MBe2BO3F2(M=Na,K) have been discovered to be nonlinear optical (NLO) materials for UV applications. They have relatively large NLO coefficients:deffis about 2.5 times as large as that of KH2PO4(&lgr;=1079 nm, Nd:YAP laser), the UV cutoff wavelengths are as short as 155 nm, and they have a moderate birefringence of nearly 0.10. Their structures are characterized by an infinite sheet which is formed by six‐membered rings [Be2BO6F2] with Be‐O and B‐O edges in common. Such structures are favorable for good NLO properties. These NLO properties make MBe2BO3F2promising candidates for vacuum UV NLO materials.