The effects of nickel content and heat treatment conditions on the creep strength of precipitation-strengthened 15Cr ferritic steel were investigated. The creep strength of the 15Cr ferritic steel was drastically improved by solution treatment and water quenching. However, over the long term, the detrimental effect of nickel on the creep strength was pronounced for water-quenched steels. The volume fraction of martensite phase increased with increased nickel content in both the furnace-cooled and water-quenched steels. The volume fraction of martensite phase in the water-quenched steel was smaller than that in the furnace-cooled type, even for the same nickel content. Fine particles, smaller than 500 nm, were precipitated homogeneously within the ferrite phase of the water-quenched steel. On the other hand, coarse block-like particles 1 $m in size were precipitated sparsely within the martensite phase. The creep strength of the steels decreased with increased volume fraction of the martensite phase caused by furnace cooling and nickel addition. The lower creep strength and microstructural stability of the martensite phase is attributable to less precipitation strengthening. To enable this steel to be put to practical use, it will be necessary to suppress the formation of the martensite phase caused by addition of nickel by optimizing the chemical composition and heat treatment conditions.