Can I use .NET 4.5 assembly as external library?

I want to use set of features, implemented in .NET Framework v4.5. Cause Unity3D works on Mono runtime(equivalent to .NET 3.5), i think to use .NET4.5 assemblies as external plugin.

Is it possible? If it possible, which runtime will be used for this libraries - Mono or assembly’s target runtime (.NET 4.5 in my case)?

no it is not possible to use a 4.5 assembly (even as external plugin). The runtime is still 3.5 even if you compile an external dll with 4.5, the basic vm/runtime features introduced in 4.x aren’t available in unity so the dll won’t load.