In order to investigate the magnetovolume instability in compound LaFe11.5Si1.5, we employed first-principles electronic-structure calcul ations b y using a full-potential linearized-augmented-plane-wave method within the gener alized-gradient approximation. The total energies of nonmagnetic and ferromagnet ic states were calculated by spin-polarization approach and the calculated resul ts showed that there is little difference between them. The fixed-spin-moment ca lculation confirmed that the metamagnetic transition was associated with low-sp in-high-spin transition in compound LaFe1.5Si1.5. Based o n the ca lculated results, we could explain qualitatively some of the experiment results.