Magnetic nanoparticles with large magnetic moments that can be manipulated with an external magnetic field, have potential uses in medicine because their sizes are comparable to biological scales. For such applications it is important to understand how their magnetic properties are affected by their size and size distribution inherently present in magnetic nanoparticles. For this purpose, maghemite (γ-Fe 2 O 3) nanoparticles with average diameters of 7.0±0.8 nm, 6.3±0.6 nm, 3.4±0.8 nm and 2.5±0.7 nm and Fe-Pt core-shell nanoparticles with an approximate core diameter of 2.2 nm were synthesized and investigated. To aid in the interpretation of the magnetic properties, the structural properties of these nanoparticles were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). For investigations of the magnetic properties, detailed ac and dc magnetic characterization is presented and discussed in terms of a distribution of particle sizes and magnetic moments. The dc magnetization measurements cover the temperature range from 2 K to 350 K and magnetic fields up to 90 kOe. The temperature dependence of the ac susceptibilities, χ′ and χ″, was measured at various frequencies from 10 Hz to 5 kHz. From the zero field-cooled dc magnetization, the values of blocking temperature have been determined and the ac magnetic data was used to determine the contribution of interparticle interactions to the observed blocking temperature for different sized nanoparticles. The measured blocking temperatures of the maghemite nanoparticles are =35 K, 42 K, 21 K, and 29 K with contributions from interparticle interactions given in terms of =0 K, 11 K, 2.5 K, and 12.5 K for the 7.0 nm, 6.3 nm, 3.4 nm, and 2.5 nm samples respectively. From the variation of with ac measurement frequency, the anisotropy constants determined for the maghemite nanoparticles are: =5.57, 7.51, 18.57, and 79.9 in units of 10 5 erg/cm 3 for the 7.0 nm, 6.3 nm, 3.4 nm, and 2.5 nm samples with a Néel-Brown attempt frequency of =2.6×10 10 Hz. The same approach applied to Fe-Pt nanoparticles yields =13 K, =5 K, =4.74×10 6 erg/cm 3 , and =5.3×10 10 Hz. For maghemite nanoparticles, the size dependence of the anisotropy shows an increase with decreasing particle diameter consistent with data of other investigators. However this dependence is more rapid than the 1/ behavior typically used to discuss the size dependence of nanoparticle magnetic anisotropy. The magnetic field dependence of the magnetization of the nanoparticles below their blocking temperature indicates negligible coercivity for the 7.0 nm, 3.4 nm, and 2.5 nm maghemite samples. However, for the 6.3 nm maghemite and the Fe-Pt samples, significant coercivity is observed with their magnitudes increasing with decreasing temperatures below and reaching 400 Oe and 750 Oe at 2 K, respectively. Above the field dependence of the magnetization of all the samples was analyzed in two different ways: in terms of a modified La...