I have 2 servers (HPE ProLiant DL390 Gen10)
I installed esxi6.7 for them.
Then they restart at irregular intervals.
The first server，about restart 5-6 times a day.
Second server，about restart 5-6 times a day.
There is no warning before restarting.
Every time it's a sudden restart.
So it should not be the problem of the server itself.
How can I find out why they are restarted when they install esxi6.7?
Your post needs moving to the ESXi area, so I have reported it.
Please log in to iLO and check the server logs.
If ESXi itself crashes, it usually doesn't restart, but displays a PSOD (Purple Screen Of Diagnostic).
As Andre is also saying , check the ILO logs. I have had issues with thermal and cooling , the fans have not leveled up automatically and I also had these reboots. Nothing related to ESXi really. I increased the fan cooling setting in BIOS , before they just was sitting at 10% and the server rebooted from time to time. After I have changed to increased cooling the fans is running at 30%-50% constantly and no reboots anymore. The inlet temperature for me is around 22-23 celsius.
The first option is to log in to the ILO on the HP servers, check the Hardware errors and your Firmware levels.
If you have not configured the ILO you should log into the BIO (UEFI) and assign an IP and password.
The ethernet port of the ILO is marked and you can see it in the Server service guide.
Connect the server with an ethernet cable to a laptop or PC and using a browser point to the ILO IP to log in.