This paper closely examines the forces of the past in Eugene O'Neill's famous play, Long Day s Journey into Night and argues that the past appearing in whatever forms is mostly negative and destructive in the play. The tragedy of O'Neill's play is deeply rooted in the past that implicitly or explicitly influences the development of both its plot and characters. By analyzing the four protagonists of the play, I will demonstrate that for one thing, the personal past reveals itself as a deterministic or fatalistic burden for the protagonists' suffering present, and the past grudges, wrongs, hurts and mistakes become ammunitions for them to blame one another as they are unable to forget and forgive, no matter how hard they may have tried. Meanwhile, the family and personal past also makes some protagonists guilty for their unforgettable mistakes; but then, the past, as nostalgia, also deceives and entraps some of them into self-indulged illusions rather than providing them with any real protecting havens in reality. Although some attempt to escape from their own past consciously or unconsciously, others attempt to preserve the past, or even to recreate it in their own illusionary realms, but the unsympathetic and cold reality deprives them of such luxurious indulgence. In the final analysis, one thing in common in all the four protagonists is that the past has appeared in whatever forms is negative and destructive rather than positive as it has either deeply traumatized some or fatally crushed others.