© 2018, TMMOB - Jeoloji Muhendisleri Odasi. All rights reserved.The enlargement of settlement areas due to increasing population emerges the construction of infrastructural and other engineering structures. The construction of engineering structures has been densely continued from the second half of the 1900‘s to the present in İstanbul. Many engineering projects are being planned for the near future. The recent structures are larger and deeper than the previous structures. Thus, the planners and engineers have faced to more geo-engineering problems related with geological uncertainties. In recent years, majority of the engineering structures in İstanbul are getting denser in the Paleozoic sequence of İstanbul along both the two sides of the Bosphorus. The Paleozoic sequence of İstanbul mostly consists of sedimentary rocks. These sedimentary rocks are crosscut by dikes with varying compositions, in random orientation and dimensions (e.g. millimeter to couple of meter in scale). The dike crosscutting the İstanbul Paleozoic rocks generate unexpected and unwanted conditions such as squeezing of TBMs, stability problems, water leakage etc. As a result, increase in time and budget of the projects occur. This study aims to define the strength and deformation properties of dikes in İstanbul under axial loadings. Besides, the brittleness of the studied rocks and their engineering behavior in practical applications were evaluated. In addition, the behavior of dikes and host rocks were compared regarding basic rock mass properties. Based on the test results, the uniaxial compressive strength and Young’s modulus of dikes and the host rocks are determined as 46-277MPa and 33-158 MPa, and 16-99 GPa and 5-57 GPa, respectively. Besides, the brittle failure of the magmatic rocks under axial loadings is obvious in the graph of strength versus strain. Once the problems encountered particularly in tunnel constructions in İstanbul are evaluated, it is defined that the differences between the strength and strain characteristics of host rocks and dikes become an important geological problems. The results of this study state necessitate the detailed geo-mechanical investigations during the construction and for the longterm performance of underground rock structure.