The 18, some from Chad, Egypt and Nigeria, were executed on Sunday in Tripoli and Benghazi, Libyan media reported.
Amnesty International said they feared the accused had not had fair trials.
The people were convicted of murder and executed by firing squad, the Libyan Cerene newspaper said.
"In Libya we fear that death sentences are handed down after proceedings which fail to satisfy international standards for fair trial," a statement from the organisation said.
Foreign nationals are at a disadvantage in the Libyan legal system, Amnesty said.
They often do not have access to lawyers and do not understand the trial proceedings because they are not translated from Arabic.
Libya executes a disproportionately large number of foreigners, Amnesty said.
Each year thousands of African migrants travel cross the Sahara desert to Libya with the hope of one day reaching Europe .
Source: BBC