ABSTRACT: The enlargement of settlement areas due to increasing population emerges the construction of infrastructuraland other engineering structures. The construction of engineering structures has been densely continued from thesecond half of the 1900’s to the present in İstanbul. Many engineering projects are being planned for the nearfuture. The recent structures are larger and deeper than the previous structures. Thus, the planners and engineershave faced to more geo-engineering problems related with geological uncertainties. In recent years, majority of theengineering structures in İstanbul are getting denser in the Paleozoic sequence of İstanbul along both the two sidesof the Bosphorus. The Paleozoic sequence of İstanbul mostly consists of sedimentary rocks. These sedimentary rocksare crosscut by dikes with varying compositions, in random orientation and dimensions (e.g. millimeter to couple ofmeter in scale). The dike crosscutting the İstanbul Paleozoic rocks generate unexpected and unwanted conditionssuch as squeezing of TBMs, stability problems, water leakage etc. As a result, increase in time and budget of theprojects occur. This study aims to define the strength and deformation properties of dikes in İstanbul under axialloadings. Besides, the brittleness of the studied rocks and their engineering behavior in practical applications wereevaluated. 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 aredetermined as 46-277MPa and 33-158 MPa, and 16-99 GPa and 5-57 GPa, respectively. Besides, the brittle failureof the magmatic rocks under axial loadings is obvious in the graph of strength versus strain. Once the problemsencountered particularly in tunnel constructions in İstanbul are evaluated, it is defined that the differences betweenthe strength and strain characteristics of host rocks and dikes become an important geological problems. The resultsof this study state necessitate the detailed geo-mechanical investigations during the construction and for the longterm performance of underground rock structure.