On the basis of detailed microscope and microprobe studies of fresh samples from some intermediate subvolcanic rocks and syntectic granitoid rocks in West United States and East China, the reason for the high Mn contents in some ilmenite has been investigated. This results show that,in the granitoids studied; 1. All the Mn-rich ilmenite is the product of oxidation "exsolusion", which indicates that low temperature (755º-450℃)，high fo2 and slow cooling velocity make up prerequisite for the formation of Mn-rich ilmenite. 2. There is no correlation between MnO(II)and Hm(II). 3. MnO(II) does not correlate with sphene contents in the rocks. 4. The correlation co0fficient between MnO(II)) and modal quartz plus alkali -feldspar is+0.82(N=21), with a confidence level of positive correlation at 99.9%. 5. The correlation coefficient between MnO(II) and modal mafic silicates is -0.70 (N=21) with a confidence level of negative correlation at 99.9%, which shows that the MnO(II) decreases with increasing content of fer- romagnesian minerals. 6. The correlation coefficient between MnO(II) and the ratio of modal mafic silicate to magnetite is -0.76, with a confidence level at 99.9%. The mafic silicates with high Mn mineral/melt distribution coefficient are the main host minerals for Mn2+，but quartz and alkali-feldspar with low Mn mineral/melt distribution coefficient do not accommodate Mn2+ in their lattices. Duiing the crystallization of mafic component-rich magma system, the more mafic silicates precipitate, the more. Ma2+ gets into the lattice of silicate mine- rats, which prohibits the high-Mn ilmenite from formation. During the .crystal- lization of leucogranitoid magma system, the more quartz and alkali feldspar precipitate, the more Mnz+ is enriched in residual melt, which.impels the high- Mn-ilmenite to be produced. The magma which has undergone a high degree of fractional crystallization for mafic silicates is not helpful to bring about the formation of Mn-rich ilmenite. Mn-content in ilmenite is also dependent on the ratio of fH2O2 of the magma system, the high fo2 and low fH2O facilitate decomposition of hornblend and biotite, and favour the formation of Mn-rich ilmenite. Mn-content in ilmenite may be applicable to discriminate between acid volcanic and subvolcanic rocks and to estimate the contents of mafic silicates and of quartz plus alkali-feldspar in some intermediate-acid rocks. The Mn-con- tent in ilmenxte may be useful in inference of fH2O and fo2 of leucogranitoid granitoid magma and in distinction of the intrusions related to porphyry copper deposits from barren ones.