How harmful for your health is 300 triglyceride level?

High: It's high, but it's not dangerous. If your LDL and HDL are reasonably ok (hdl tends to be low when tg is high) and if you don't smoke, don't have dm, don't have htn, and don't have a strong family history of premature hd, it's of no concern. It can be treated with lifestyle changes: see - http://www.Webmd.Com/cholesterol-management/tc/high-triglycerides-overview.