28 May 1993
Several attempts were made by Eritrea to gain recognition from the United Nations (UN).  Habte Selassie who represented the Eritrean People’s Liberation Front (EPLF) at the UN argued that the UN’s resolutions on decolonisation were to be applied to Eritrea. The Ethiopian Government agreed to support Eritrea’s right to self- determination. Ethiopia wrote a letter to the UN Secretary General. The UN approved an Observer Mission (UNOVER) to ratify Eritrean’s Referendum. Soon after gaining its independence after 30 years of struggle against its former ruler, Ethiopia, Eritrea joined the United Nations (UN) as an official member on 28 May 1993.
References

Killion T. & Connell D. (2011), Historical Dictionary of Eritrea, (Scarecrow Press), pp.523-524|Desta A., The  Permanent Mission  of Eritrea to the United Nations, [online], Available at:  www.eritrea-unmission.org  [Accessed: 6 May 2014]