In tlıis research, tlıe dual and triple relationships ol' diam eter outside bark and site index to double bark tlıickııess, diam eter inside bark, b ark cofficent and diam eter outside b a rk iııcrem ent are investigated in beech forest in Turkey. Besides, the mean b ark ratio and coel'ficent of beech trees are also determ incd using the relation between diam eters outside and inside bark. These relationships are fixed depending on the increm ent cores data of 1340 beech trees lıaviııg different diam eters, Iıeights and conditions of grovving site. The relationships of regression ıııodels are balanced so as to fit to the dispersion of the points dotted on the graplıs. The non-linear regression metlıod is used in regression analyses. Iıı the research, the tables of double bark tlıicknesses and b a rk coefficents lıave been arı anged depending on diam eters outside-inside b ark and site quality degrees, in addition to the tables of double b ark tlıicknesses by diam eters outside bark.