Car is overheating after replacing radiator

Make sure the cooling system is fully purged of air pockets. If you don't have a coolant funnel park it on an incline and let it run with the rad cap off and the heat on full blast. If that doesn't help and your radiator broke along the top...hopefully for your sake it's not a bad head gasket.

I also agree with Mountain Auto! Also make sure there is enough coolant/water in the entire cooling system like Mountain said air pockets will make you think it's full and it won't be make sure the thermostat isn't sticking either