My daughter age 5 month having rapid heart rate about 240 per minute.is this dangerous? mybabys growth and other activity are normal.

Yes, it can be: She may have a condition called SVT. The infant can tolerate this for a short period of time, and then becomes very ill very quickly. You have a short time to get her heart regulated before it could start to fail, sometimes rapidly. Sign off and go to ER now. If you live in a city with a Children's Hospital, go there. If not, nearest ER will be fine - they will transfer her if needed.