Speaking after assuming charge, he said Sri Lanka needs to be wary of challenges to national security as the island makes progress towards achieving national reconciliation.
"As we make progress towards political reconciliation in an environment of political and economic stability we are being made to face national security challenges which are coming up with a new face," said Ratnayake.
Ratnayake, who was was enlisted to the Regular Force of the Sri Lanka Army in 1980 as an Officer Cadet, became the second Commander of the Army after the LTTE was militarily defeated four years ago in 2009.
Sri Lankan troops' conduct during the final phase of the military campaign against Tamil Tigers has come under scrutiny of international rights bodies including the UN.
The UN's Human Rights Council has passed two resolutions urging for accountability for alleged rights abuses and means to achieve national reconciliation with the Tamil minority.