I have played COC on iPad. Can I use the data in my iPhone?

Yes.
You can use the data of your clash of clans into any device that you also want to play it with.
In transferring data from your iPad to iPhone, you have to have both of the devices with you to do it.
If you do not have the clash of clans on your iPhone yet, you may want to download it first from the app store.
Once done with the download, go through with the tutorial first.
There is no shortcut to this one just yet.
Open clash of clans on both devices on game center to verify your connection.
Then on both devices, click on the link a device tab under the settings option found on the lower right of your game screen.
On your iPad, click under the “this is the old device” tab.
It will then give you a code that expires within 5 minutes.
This kind of code is private and should not be shared to anyone.
On your iPhone, click on the “this is the new device” option.
It will then ask you a code generated from your old device.
Enter the code given from your iPad.
Once the code is entered, data from your old device will appear on the new device and so you can start playing your clash of clans from your iPhone just as how you played it on your iPad.