Bicycle disc brake rotors It is time to change my bike's hydraulic disc brakes . Now I have a Hayes Stroker Ryde system , but I want to order a Shimano M395 instead . The question is that I don't know If I have to order the rotors with brakes or not . Both brakes are for 160 mm rotor but they don't look alike . This is my hayes brake rotors !enter image description here And this is the rotors of shimano m395!enter image description here

As long as you existing rotors are in good condition and thicker than 1.6mm (minimum safe thickness) you should have no issue with using them with the Shimanos. There doesn't seem to be an industry standard specification for rotor thickness but it is generally taken to be between 1.8mm - 2.0mm. The new callipers will self adjust to the rotor width.

Clean your existing rotor up with some rubbing alcohol (or similar solvent which doesn't leave a residue) before installing the new brake system to ensure there's no contamination.