Abstarct: The study area as a part of Kahak- abbasabad Eocene magmatic belt, is situated in the northeast edge of Central Iran Zone, extended from davarzan in the west of Sabzevar to Mayamey in the east of Shahrood and in the southern part of the Sabzevar ophiolitic belt. Rock Units of the area include an alternation of volcanic rocks with basic (trachybasalt and olivine basalt) to intermediate (trchyandesite, trachyandesitic basalt and andesite) composition and volcano–sedimentary rocks (varieties of tuffs, breccia, agglomerate, sandstone, Shale, conglomerate, nummolitic limestone and tuffuceous limestone) belong to Upper-middle Eocene, and sedimentary rocks of Oligocene and Quaternary. The volcanic rocks of this belt include a continues fractional seri that originated from a common parental basaltic magma. Since the basaltic and andesitic lava flows are of pyroxene porphyry, plagioclase porphyry and pyroxene- plagioclase porphyry, then they are favorite samples for crystal size distribution (CSD) studies. The CSD in these rocks have been investigated for the first time in this study. According to calculations baxsed on slope of CSD diagram, plagioclase phenocrysts of the volcanic rocks have been grown in magmatic chamber in range of 86.20 to 118.01 years with nucleation rates of 1.48×10-10 to 1.56×10-10 mms-1. Also, according to calculations baxsed on slope of CSD diagram, pyroxene phenocrysts of the volcanic rocks have been grown in magmatic chamber in range of 1.02×10-3 to 2.01×10-3 years with nucleation rates of 0.55×10-3 to 2×10-5 mms-1. The crystal size intervals in the studied andesitic and basaltic rocks have been about 0.05 to 5.62 mm and 0.03 to 0.16 mm for pyroxene and plagioclase crystals respectively. Also, volume proportion estimations for each thin section of the basaltic and andesitic rock samples were variable and range from 2.23 to 11.37 volume percents for plagioclase crystals and from 10.02 to 22.93 volume percents for pyroxene crystals. The CSD diagrams resulted from quantitative studies of plagioclase and pyroxene crystals reveal the fractionation and accumulation of crystals, coarsening and mixing of the different crystal populations in the andesitic and basaltic rocks of this area. These Evidence along to petrographic features such as glomeroporphyric, microlitic porphyry, porphyry and disequilibrium textures like zoning, rounding, golf corrosion and sieve textures in the plagioclase and pyroxene crystals indicate that magma mixing or mixing of the different crystal populations and magma fractionation were the effective processes in the formation of the igneous rocks of the area.