This paper describes the use of municipal solid waste incinerator(MSWI) ashes to manufacture synthetic lightweight aggregates using an innovative sintering technique, named as 'inner exothermic sintering(IES) method', characterized by co-treatment of sewage sludge as an inner heating source and bloating agent. The results of sintering experiments indicate that the added sewage sludge has successfully acted as an inner heating source d a bloating agent, and thus could be manufacture of a double layered MSWI ash aggregate with fusion-bloated inner part and non-fusion dense outer surface. Futhermore, in this process, the fly ash also be co-treated up o 10% without any particular change of the sintering condition. The synthetic aggregates prepared by IES methode are tested according to US regulation I and fulfill regulatory limits, therefore, confirming their safe applications, although the results show some higher leaching from sintered aggregates than from melted slags. An overall conclusion from this is that the IES method represents an environment friendly and a safe solution :or stabilising and converting MSWI ashes and sewage sludges into useful products.