This research presents a method for assessment of socio-economic development of country (in the case of Bulgaria) in the context of sustainable development. A new complex index was estimated, called Sustainable development socio-economic index (SDSEI), which was designed on the base of eight representatives for the sustainable development indicators define by the National Statistic Institutes of Bulgaria. The socio-economic development of Bulgaria has been studied the period 2003-2015 year and was identify years in which it could be define as sustainable and years it could be defined as less or unsustainable. Conclusions about applicable of the used method and about socio-economic development of Bulgaria were made.