Two groups of porous metal bonded diamond composites (PMBDC), with and without rare earth (RE), were compacted and sintered under the same conditions. The effects of RE on the transverse rupture strength (TRS) and fractography of fracture surfaces (FFS) of PMBDC having different porosity were investigated. From the TRS and FFS’s perspective, the composites with RE are better than those without RE. Furthermore, the result suggests that there is a critical porosity where there are very apparent effects on the TRS and FFS. If the porosity of the composites is less than the critical value, the effects of RE increase with the porosity; if the porosity is larger than the critical value, the effects drop with the porosity instead.