<P><B>Abstract</B></P> <P>Two-stage thermophilic solid state anaerobic digestion (SS-AD) processes for biohythane production from <I>Chlorella</I> sp. biomass was investigated. The hydrogen and methane yield from fresh <I>Chlorella</I> sp. biomass was 124.9 mL H<SUB>2</SUB> gVS<SUP>−1</SUP> and 230.1 mL CH<SUB>4</SUB> gVS<SUP>−1</SUP>, respectively, at 16% TS with biodegradation efficiency of 34%. Hydrogen and methane production from hydrothermal pretreated <I>Chlorella</I> sp. biomass was 190.0 mL H<SUB>2</SUB> gVS<SUP>−1</SUP> and 319.8 mL CH<SUB>4</SUB> gVS<SUP>−1</SUP>, respectively at 16% TS with biodegradation efficiency of 47%. Pretreatment of <I>Chlorella</I> sp. biomass with hydrothermal follow by two-stage thermophilic SS-AD improved 150% biogas production comparing with fresh biomass. <I>Clostridium</I> sp. was the main bacteria responsible for hydrogen production. <I>Clostridium</I> sp. and <I>Ruminococcus</I> sp. was main bacteria in methane stage, while <I>Methanoculleus</I> sp. and <I>Methanothermobacter</I> sp. was dominated archaea in methane production process. <I>Chlorella</I> sp. biomass with hydrothermal pretreatment has great potential for hydrogen and methane production via two-stage thermophilic SS-AD.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Two stage thermophilic solid-state anaerobic digestion of <I>Chlorella</I> sp. was investigated. </LI> <LI> Pretreatment of <I>Chlorella</I> sp. with hydrothermal improved 150% biohythane production. </LI> <LI> Biohythane yield from <I>Chlorella</I> sp. biomass was 190.0 mL H<SUB>2</SUB> gVS<SUP>−1</SUP> and 319.8 mL CH<SUB>4</SUB> gVS<SUP>−1</SUP>. </LI> <LI> Dominant bacteria in hydrogen stage was <I>Clostridium</I> sp. </LI> <LI> Dominant archaea in methane stage were <I>Methanoculleus</I> sp. and <I>Methanothermobacter</I> sp. </LI> </UL> </P>