Pre-existing BCR-ABL kinase domain mutation leads to Imatinib resistance. Retrospective analysis of 50 patients of Imatinib resistance was done in GCRI, from January 2014 until May 2014. Allele Specific Oligonucleotide –Polymerase Chain Reaction (ASO-PCR) was performed on Genomic DNA of peripheral blood mononuclear cells (PBMCs). 47( 94%) were in Chronic phase, 2(4%) in accelerated phase, 1 (2%) in blastic crisis. Median duration of Imatinib was 48 months. 43/50 had one or more than 1 mutation, T315I mutation in 5 (10%) patients, M351T in 32% (16/50) & F311L in 8. We report M351T as the most common detected mutation 32% followed F311L 16% as against T351I , frequently reported. These mutations may be pre-existing and may have preceded T351I , which is acquired later.