We design a quasi-L2 (qL2) photonic crystal nano-cavity (PCN), which has a very small mode volume Vm of 1.6(λ/2n)3 and a reasonably high Q value of 15,000 theoretically. By manipulating the radii of air holes, the mode spacing is extended and a single defect mode operation within the gain spectrum of InAs QDs is achieved. A room temperature operation InAs QDs PCN laser with an ultra-low threshold power of 0.8 μW, corresponding to effective absorption power of less than 100 nW, is realized. Fitting the measured data to the theoretical estimation by coupled rate equation model, a high spontaneous emission coupling efficiency of 80% is obtained. We ascribe this encouraging performance to strong Purcell effect as a result of a small Vm and high coupling efficiency of the cavity.