Objective: To investigate the relationship between H3.3K27M mutation and H3K27me3 protein expression by immunohistochemistry (IHC) and to discuss the correlation between K27M-H3.3 mutation and thalamic glioma prognosis. Methods: We Collected the clinical documents and data of 24 thalamic glioma patients who received surgical resection from January 2013 to December 2016, including sex, age, initial symptoms, clinical manifestations, CT/MRI images and pathologic test results. We recorded each patient’s surviving data, including progress-free survival (PFS) and overall survival (OS). Finally, we separated them into experimental group and control group. The sections were tested by IHC with H3K27me3 antibody in order to estimate H3K27me3 protein expression. Analysis of prognosis was investigated by the SPSS V20.0 software. Results: The levels of H3K27 trimethylation (H3K27me3) were reduced globally in all the 11 H3.3K27M patient sample sections, while all the control group sections presented normally, lack of reduced or even deleted H3K27me3 areas. There was no significant statistical difference in OS and PFS between these two groups. Conclusion: The K27M-H3.3 mutation can severely lead to H3K27me3 reduction or even deletion, due to the reduced trimethylation of the 27th amino of H3 histone by H3.3K27M mutation. The prognostic factor of K27M-H3.3 mutation is still unclear.