The hydrolysis and acidification abilities, dewaterabilities and sludge reduction of waste activated sludge (WAS) under four types of alkaline conditions, which were KOH, NaOH, Ca(OH)2 and Na2CO3, were investigated. Temperature and pH value were controlled at 35 ℃ and 10 respectively for all alkaline conditions investigated. WAS demonstrated different hydrolysis abilities, acidification abilities and dewaterabilities under the four types of alkaline conditions. The order of WAS hydrolysis abilities, acidification abilities, ammonia and phosphorus release, dewaterabilities under the four types of alkaline conditions were Na2CO3 > NaOH ≈ KOH > Ca(OH)2,, NaOH > KOH ≈ Na2CO3 > Ca(OH)2, Na2CO3 > NaOH ≈ KOH > Ca(OH)2 and Ca(OH)2,>Na2CO3 > NaOH ≈ KOH, respectively. Volatile suspended solid (VSS) reduction of WAS was the best under Na2CO3 condition; while under NaOH condition total suspended solid (TSS) reduction was the best.