Pure phase of LaP3O9Eu3+ phosphors with orthorhombic structure was successfully synthesized by a polyacrylamide gel process. The crystal structure, thermal behavior, and optical properties of the as-synthesized samples were investigated by XRD, TG-DTA, FT-IR, and PL. The effects of some synthesis conditions on the structure and properties of samples were also discussed. The results reveal that the annealing temperature and the ratio of (La+Eu) to P in precursor solutions play a key role in determining the structure and luminescence properties of products. The single-phase orthorhombic structure LaP3 O-9:Eu3+ phosphors can be obtained by annealing at 750-850 degrees C. The LaP3O9:Eu3+ phosphors emit a dominating and intense line at 618 nm, and exhibit improved photoluminescence properties compared with the samples of LaPO4:Eu3+ crystals. The concentration quenching effect can be observed with increasing Eu3+ concentration over 40 at.%.