In essence probably not, but it depends what you're trying to do.
By European spec, I assume you mean an PN rated flange. A direct replacement is not physically possible as the design of the two types of flanges is not the same and they don't match. you can sometimes use a flange adaptor where a plate with a hole in it has two different sets of stud bolts welded to the different sides, but they look terrible and the strength of them is often questionable.
However, if you replace an entire flange set or flanges and valve, this can be accomplished providing that the design pressure of the system is equal or lower to your now new flange rating. Eg if a design pressure is 16 barg or less, you could replace a class 150 flange set with a PN 16 rated flange set. You might need to use the next rating up which is higher than the ANSI class rating to do this.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Thank you for your reply LittleInch.
Maybe I wasn't clear with the question.
Can You connect a valve with European DIN Spec, with a flange ANSI 150? Will the bolt holes match just to fit the bolts and achieve a connection?
It's been tried but is NOT safe and is NOT approved for use. The usual method is to jury-rig the two flanges with a sub-sized (smaller) bolt or stud wedged at an angle between "almost-lined-up" metric and ANSI bolt holes.
Best solution is to determine the needed pressure and service ratings, then make a very short (1 - 2 feet/half-meter or less) transition piece with an ANSI flange on one end and the EU flange size on the other.
This then bolts to the valve or flange safely on both ends.
