You Do Need power plug adapter while traveling from Serbia to Thailand.

Here is Why You Need An Adapter

The outlet and plug type used in Thailand are A, B, C and O and in Serbia are C and F. This is how they look like.

Serbia

In Serbia, outlets of type C and F are in use.

Type C

Type F

Thailand

In Thailand, outlets of type A, B, C and O are in use.

Type A

Type B

Type C

Type O