This paper deals with three main effects of Avicenna on the Islamic Astronomy. First, he, methodologically, made a remarkable distinct between Astronomy, as a proofed and mathematical science, and Astrology, as a conjectural knowledge, and hence, he made a principal separation between Astronomy and Astrology, while these two assumed to be permanently as a united whole throughout the Babylonian, Greek, and Hellenistic periods. Second, his defense of the Aristotelian philosophy against the experimental doubts of Biruni. Third, the effect of his peripatetic ideas on the astronomical activities in the first period of Maragha observatory, namely, the scientific circle established by Nasir al-Din al-Tusi. These effects bear some positive and negative consequences on Astronomy; the first may be assumed as a step to forward, while the two latter ones diminished the power of Astronomy as an experiential science and made it unable to change the customary, dominant ideas of the natural philosophy, or, in some places, reduced it as a purely geometrical science.