Hamza Nawaz, 24, and Mohommod Nawaz, 30, were stopped by police in September last year as they arrived at the port in Dover from Calais.
Officers who searched their car allegedly found a balaclava, "heavy-duty clothing", six mobile phones, a sim card inside a Koran and ammunition for an AK-type.
It was claimed the pair had driven from their home in Stratford, east London, to Calais, then flown from Lyon to Turkey, and had travelled on from there to Syria.
At a hearing at the Old Bailey, both men admitted conspiring to attend a place used for terrorist training between January 1, 2012 and September 16, 2013.
Mohommod Nawaz also admitted possessing ammunition without a firearms certificate. The same charge against his brother was ordered to remain on file.
The brothers, who were remanded in custody, will be sentenced on June 9.