WhatsApp Messenger uses the same internet data plan as web browsing and email on your phone. If using your cellular data while roaming normally results in additional charges, you may be charged extra by your mobile provider for using WhatsApp while roaming. Contact your wireless carrier for details about international roaming countries, SIMs, rates and policies. If you do not have an international data plan, turn off Data Services While Roaming to avoid data roaming charges.
To confirm that your data roaming is disabled go to:
Settings > Network and Connections > Mobile Network and make sure Data Services while roaming is set to Off.
Provided you have Wi-Fi internet access and active WhatsApp service, WhatsApp should be free to use over a Wi-Fi connection.
If data roaming is enabled on your device, we suggest that you set Media auto-download to Wi-Fi only to avoid extra data charges. To learn how to configure media auto-download, please read here.
While traveling, we recommend that you set your WhatsApp status to "I am currently roaming abroad, will reply when I am on Wi-Fi". This way your friends will know why you are not answering right away.