I used omron MY2N control relays to drive the DC24V current to turn on/off the hydraulic valves. I found that the spark will occur between the contact during turned off the control relay. Is there any countermeasure for me to dissipate the spark? Thanks

Yes, it's called "inductive kickback".

http://www.coilgun.info/theory/inductivekickback.htm

Thanks you very much. Anyway, now my connection is +24V-->Contact-->Valve-->0V. Is that i need to change to +24V-->Valve-->Contact-->0V before add in the additional diod between the 2 terminal of hydraulic valve? Is there any other way to do it? And, how about for AC 220V?

Hello Jacky

There are a number of options that can be used.
One option, is to connect a diode across the coil so that it is reverse polarised when the voltage is applied to the coil.
This will stop the arc, but will dramatically slow down the reduction in current flow through the coil. This can make the valve operation sluggish.
To speed up the release of the field in the coil, you can connect a resistor in series with the diode. This will allow the field to collapse at a greater rate, but will also increase the back emf generated by the coil.

Another method is to connect an MOV across the relay contacts. This will limit the voltage across the contacts and reduce the arcing.

A snubber network across the contacts will have a similar effect, but wil be less effective.

Best regards,