interest in biorhythm mathematical model has been growing in recent years as a resuit of several active and at times controverial, areas of research.the day/night or circadian rhythms with light/dark cycle is well known and reasonably well understood.It is well documents for this research is about a modified mathematical model for neurospora biorhythm. Biological systems have long been proving by theoretical models that useful in clarifying the conditions in which periodic phenomena.the previous model for oscillations resulting from negative feedback loop was due to Goodwin.the purpose of the present article is to examine both of the biochemical variables and the light/dark cycle by the sets of differential equation system.